Instead of seeing things in terms of "good" and "evil", try to see them in terms of things that work out for us, and things that don't.

Murder for example, doesn't benefit anybody. It makes people's lives miserable, and nobody ever got happy from killing anyone (although they probably often thought they would). Only if you take out someone who is about to take the lives of many others, does murder seem to be beneficial.
Even those who don't seem to feel any remorse when commiting first degree murder, are in that mental state due to a lack of love in their life. When somebody commits an act society considers evil, it is because at that particular time, they thought that it would be a decision that would improve their life. Even if somebody loses self-conrol in a fit of rage this applies. Because nobody ever does anything that they consider as wrong, at least not at the moment they do it.

Evil is not the opposite of good, but desperate manifestations of a lack of love. Which is what we all ultimately want - love. Good is the most effective way of finding it, evil is self-destructive. But if you have a lack of love in your life, whether due to how you were raised or because you closed yourself so you didn't receive what was sent to you, how could you possibly know that good is the way to find happiness?
Which is also why you can't possibly fight evil with evil.

A good example of how this works, is sibling rivalry. You know how kids get jealous when they get a new baby brother or sister? That's because they are affraid they will lose the love they get from their parents, and that may come to the point where they start hating their bro/sis, even hurt them. Why? Because they think that will make their lives better, oblivious to the hurt this wil cause. They see somebody else get the love that they are affraid to lose, and their immature minds figure that things will get solved if that new person will dissapear.
I'd rather call that ignorant of how things work, than evil.