I used to wrestle ALOT with this topic. learning how young we are as a species, learning how stupid the vast majority of people are, and understanding that we do not have freewill really helped me to comprehend some of the "why?" behind violence. Also it is good to remind yourself that we live in the safest time period that has ever existed in recorded history.

You like so many others are coming at this the wrong way.
Human beings are not developing violent tendencies, they are simply expressing their repressed nature, the same nature that governed human society and evolution for almost all of the existence of the species.
We have in the west had almost five generations grow up since WW2 in relative peace, prosperity and the expectation that we will not have to take up arms to feed or protect ourselves. The horrors of two international bloodbaths made those who lived through them determined never to endure that again, but today's generation looks back and sees not horror but excitement, simple solutions to everyday problems and a time when no life that was not your own had value and they long for it.
When the child screams profanity in the street, the urge to simply set an example by killing him, rises too easily in the mind of the one who feels helpless in the face of barbarism and a world without honour or even manners.
Our base nature has been suppressed channelled and pointed toward sport, competition, business, economics, education.
But still there is the need to feel worth and superiority to someone else, anyone one else.
Then as the population has grown out of control, which each subsequent generation expecting more and more, at the expense of one part of humanity in favour of another, to the point where now poverty is seen in insufficient plenty, starvation is seen in obesity and attack is seen in simple presence and insult in disagreement. Personal Consequence is not even considered and seen as something not applicable to the perpetrator and self.
Where once people fought for food they will fight over luxuries and call then necessities
For many the last resort has already become the first reaction.
Expect to see in the coming years more wars, civil unrest, street violence, cannibalism and the reinstatement of capital punishment on a scale not seen in 200 years.
Death will become a way of life that people will simply accept as normality and the calls to "exterminate" the "other" grow and put monsters in power.

Human beings are not, good, not nice, they are great and potentially wondrous, but they need control, purpose, focus, discipline and above all a sense of worth, this is the one thing religion has never given them without a call to war, but a true secular society might IF it can decide to do it.

What Freud said. Seriously though, the human creature seems to get pleasure out of humiliating or discriminating against others..