Well I think it is mostly premeditated so get used to the idea beforehand and/or rationalise in some way. This is probably why when told to kill so people killed surrounding someone before they killing target. Bin Ladin put his youngest wife in front of him in his last moments to try to protect himself. He was shot twice in the head as to avoid shooting that woman as well. The man who killed him never got involved in the army/marines (I don't recall which) and started a company for people who still need to pay bills after leaving one of the defence forces. I can't know but I think it is something you may not feel strongly about(without insanity or heartlessness), have to make your peace with those actions and those who can not make peace with their actions. Or Post dramatic disorder which Discovery showed a deliberating use of.

I've never killed anyone but I can't tell a situation. It is situational.
There is not stepping in after someone is at the beginning of a suicide by ways that may be able to be stepped in. Hanging, significant cutting or accidents like drowning. Someone who pulls a plug. That is bystander.
Assisted suicide - Do not Resuscitate If you see a tattoo that says that and you do so you can be sued. The injection so someone in pain, who wants to be around family without them being bystanders.
War - To protect people a group of people or give aid. This can give someone a sense of purpose, or it can be the hell is better than war because hell is made out of purely sinners and war is full of bystanders. That might get into your head. There is friendly fire, firing on a hostile group or assassination. Also regular battle I suppose. There is the argument that there will be a vacuum if that group held power only to be replaced by another or killing of just one person can stop an atomic bomb being pressed.
Emotional killing of friends or family - Usually family members in fear (2012 suicide and murder because of belief it would be better than an apocalypse), anger, victim. A friend who might share personal details or fear that you'll be killed it you don't.
Shootings? - I can't say. I wouldn't say that its purely mental issues though the recent school shooting by a teenage boy was a messed up kid with red flags, solutions, Anger, dissolutions better off dead.
Bombings - Mostly by adults but motivated by a particular idea in their heads or group. Racism, lesser religious people, revenge of some kind like FBI or IRA. In this case this would be mostly considered 'necessary' and/or deserved it.
Well that's one side of it.
Genuine self-preservation - When a dangerous group comes onto property without identifying themselves only shooting when being shot at/threatened. Someone who stays while backing into some form of safety while others around him/her are going
Aid - Someone who has tried the most to decrease pain or despite psychology, friends, painkillers doesn't find life worth living. Some terminally ill would prefer this. A doctor, a friend someone who hasn't been told but really feels this to be true.
Quick indoctrinated behaviour such as you shoot a black kid rather than bring him or her in for questioning. Also friendly fire intentional or not can fall into this. They shot first... Or had a weapon at all. This is organisational which is some of the worst.
The instinct to shoot in chaos? Fear, anger, protective, just what you're supposed to do? I don't know this is guesses when it comes down to it.
This is guesses. I've never known any murders that I know of... I mean maybe Pete he was in Vietnam but I have never picked his brain and don't know if he ever left trenches and stuff like that.

Criminals never think they will be caught.

IMHO, the part of the human brain that makes us to stop when we are going to do something wrong....is not working in a criminal's brain...or is not even there.

when I was a child about 8 years old we had a large semi detached house our neighbours son killed his mother by beating her to death as well as her budgie whose neck he wrang and then laid on her breast - he had mental health unwellness, and was probably sent to his very edge by his mum and budgie skriking . He was picked up by the police at a seaside resort trying to pluck up the will to kill himself and he was sent to 'Broadmoor'. I can't stop remembering him as a kindly soul not quite with us but extraordinarily kindly and quiet.

I think on some level a murderer does not think in terms of innocence. It's all about them an there is no empathy. Drug users needing a fix will just kill whatever is in their way. Many have so little regard for life it is used as a right of passage in a gang. Empathy can be taught but if you are raised without it you may never know it.

I would think too many drugs, just guessing.

I have met two people that have killed in my lifetime. One seemed to be quite a decent guy, affable and friendly. Only after knowing him a while did he let slip the fact that he was out on licence. (a parol term for convicted killers). I can only assume it was a mistake of youth, a fight taken to exteem or some such.

The second was a woman who had killed her husband. By all accounts he was a guy who deserved all he got. After years of abuse she had finally snapped and stabbed him to death. Her nickname was Paranoid Penny (it being the custom of the community to give people such nicknames). She had a strange and wild look in her eyes, like that of a cornered and frightened animal. I only met her once and that was in a room full of guys sitting around chewing the fat and some such as guys do. When she entered the room it fell silent. Gone was all the bonhomie. There was not even a trace of flirting that would usually accompany the entrance of a woman into an all male conclave. Insead every guy sat quite silent with crossed legs until she left.

The third case is by far the most tragic. I never met the killer in question but I knew his father and he told me the tale. His son was mentally ill with paranoid schizophrenia, here are his dads words as best as I can remember them. " We knew that he was going to kill someone, it was obvious. Both me and his mum went and pleaded with the doctors/police anyone in authority who would listen and said " Hes going to kill someone " but they could not or would not do anything. And then he killed someone and now hes in a hospital for the criminally insane "

I think circumstance is a big decider. Whilst in the infantry I could kill easily relative to how I feel about the subject now. Circumstances, including training, were different. I should imagine murder works a lot the same. Is the murderer lashing out in a panicked escape when surprised, or walking up behind a Canadian tourist and shooting them in the back of the head? Is it a victim of an abusive relationship killing the abuser in order to find some escape? I would think the motives change from crime to crime as the circumstances change.

There is a term called "Crime of Passion" also self defense gone wrong, undue force, mental illness, alcohol, drugs.
I like to think I haven't terminated anyone who didn't deserve it. (joke)
In my mind, some people are evil.

Why you are asking us? How many murderers you think will be here, ready to tell you. You should go to the source. The are the only real answer. Don't do shortcuts!!!! You are asking the wrong people. I hope.

Because if they murdered guilty people they'd be vigilantes.

Murder as a crime takes on its meaning based on circumstances surrounding the murder. There are crimes of passion where murder is not the intent but is the result. There are premeditated crimes where the murder is intended and planned before hand. There is state sponsored murder which may or may not be a crime. This would include things like police shootings, military operations, and death sentences. I believe that killing another human in and of itself is wrong. I don't support the Death Penalty and do not consider a fetus to be human until birth or attaining its ability to survive outside the womb without artificial supports. Suicide is a personal choice and euthanasia should be legal.

Speaking from years of experience with criminals and with murderers of all types including those who kill during the commission of a crime such as an armed robbery, those who kill a family member, acquaintance, coworker/supervisor, and serial killers. The vast majority are not "criminals" as we generally describe them. A criminal commits crimes as a lifestyle. The murderers that a criminal commits are gang related or a dispute generally about money or drugs, or contract murders. The characteristic almost all murderers have in common is a psychopathic personality with no capacity for empathy or remorse. Many who are released from prison after serving time for murder do not kill again. I suspect that that is more about avoidance of the consequences rather than a change of heart. Unfortunately, it seems that we are a long ways away from determining the causes of psychopathic personality disorder and potential treatment.

I think that is an extremely complicated question, and one our greatest thinkers should spend more time on. I also think almost every war I've studied could be comparable. None of these wars are about what they are advertised to be about and in essence mass murder.

We are all capable under the right circumstances but on the other extreme some people love it

It's all rather relative, isn't it?

While you're taking the time to think all this through - in the chaos of a crime in progress - even one that has been planned , it seems the only thing the criminal is thinking (if, anything at all), is to get what they came for - whether it's money for the next fix, money for whatever else, to rape and plunder, or to appease their rage momentarily. Some, pumped up on drugs are totally out of it, and barely respond to being tazed.
Thinking about consequences, or the loss of life ? Not likely happening.

I don't even think all criminals are necessarily evil or "malevolent", which is why you sometimes hear the term "crimes of passion" - after which the perpetrator goes back to relative normal, and is as surprised about what they did, as all those who know them.

About 30 % or more murders in the US go unsolved when last I saw. I personally find murder to be senseless and disgusting. I watch a lot of true crime and it seems to me that people murder for a variety of reasons. Everything from twisted pleasure to a feeling of desperation. Often people kill because they are extremely emotional. Other times it appears to them to be necessary to achieve their goals. Occasionally someone just wants to kill. In any case it is objectivly wrong. It is part of human nature. Violence was and still is used as a way to ensure your blood line continues.

I believe they lack the mental capacity to understand empathy. To become a murderer must be an incredibly long process with all sorts of crossroads. As for it being human nature, yes it has to be. All sorts of animals murder their own and even their own family members.

I don't imagine they give it much thought. If they did, they probably feel justified in their actions.