I know not why people do evil.
There could be many reasons, including abuse in their childhoods. I saw the saying that God already forgiven everyone for their bad/evil things. I hope that is true, because I also did some (very) bad, dark evil things in my life.
I received a lot of hate during my childhood, both from my father and my schoolmates/classmates. I was a very emotional person as a child, but the above mentioned things made me evil on some degree.
I forgiven all of the above, hopefully everyone else (including God) forgive me.
Nowadays the world (at least this world) is full of evil things and doings.
Many people have bad lives, which they do not deserve.
I really hope that God will forgive and help both this world and me.
I think the most important reason is that we feel whether or not something evil occurs. Knowledge about the world around us also plays a significant part in this regard. We can't know something is wrong if we're raised to believe nothing is wrong with what we do.