Some notable quotes on evil:
"Hell is other people."-J. P. Sartre
"Hell is yourself."- Tennessee Williams
"Evil does not have a stomach-turning stench, nor does it signal its presence with palpable cold and darkness. (Unless Stephen Miller enters the room-my inj.) We do not fall haplessly, not does it entrap (possess) us. Rather we often act willfully in complicit ways." -Nel Noddings
"Hell is other people" was meant by Sartre primarily to convey the notion that you can't truly know yourself without the uncomfortable judgments of others, because you are generally blind to your own bullshit. However, it works in the more straightforward way that people tend to understand it, too, sadly. There's a tension between the fact that we're hyper-social and therefore crave interaction with others, as well as their affection and validation; and the fact that, in general, people suck in various ways and so cannot be counted on. Depending on Other People is at least potentially giving up your power and self-determination to their whims. And if you pay too much attention too long to their judgments, fecklessness, cruelty and disloyalty, you start to doubt yourself -- the only person you really CAN count on.
I do not think Sartre was talking about evil either. Evil is first and foremost a theological concept and therefore means whatever anyone decides that it means.
I prefer to talk about things like harms, dysfunction, and sociopathy rather than "evil".
Despite everything I've just said about people, there is a flip side to the coin, which is that we do end up effectively negotiating a societal morality that tends, in the long run, to sustainably promote civility and fairness and empathy in our interactions -- to appeal as it were to the better angels of our nature.
That is why, as Noddings says, we are often complicit and willful in harming others, yet, there is a basis for transcending that, and having something besides self-loathing for being human.
I think that all these quotes have something in common: the evil men do to others, and in many cases, the evil we do to ourselves.. We also have to deal with the harsh nature of being, from illness, traffic accidents to earthquakes. I think we need a 'Be Nice' day all over the world to the value of politeness, kindness, patience etc. in just one day. The apostle Paul asked Christians to adopt this way of life on a permanent basis in his letter to the Galatians. Christians can't do this and neither can we - it's just so very . But we could make an effort.