Those are two different questions. The concept evil is a human construct and would not even have been made had it not already been occurring sufficiently often to make an issue of it.
Why do folk kill ? A whole raft of reasons. Who is to say why the mind works as well as it does at all. It's a bit of a miracle it isn;t always going wrong, although I dare say the species wouldn't have survived if it were too common an activity.
Until you have experience a mind that doesn't work too well, according to society's definitions, then it is a difficult thing to understand. Suffice to say that knowning something, and finding the ability to act according to that knowledge, are different things.