I don't believe in God or the Devil. I believe in good and evil for sure though.
I believe everyone is born with a set of commandments from within. If you broke your inner rules then it would tear open holes within you that evil filled. Some people call it guilt, but guilt is something that other people impose upon you.
The feeling I'm talking about comes from within and it's impossible to heal until you correct the wrong you have committed. You all have felt it: when you did something that little voice was telling you not to, but in the end everything turned out OK, right? WRONG. Yeah you can push that feeling away but it's still there and the next time you do something wrong, it will grow and continue growing until you don't even notice it anymore.
You make yourself believe a lie like "it doesn't matter" or "this was the best choice I could make" when deep down you know that's not true. This acceptance of the break in your moral code makes the hole in you even wider and that's when you start treating evil things like it's fine. It may start small, a lie here or there, or you cheat on a test, or you could have just stole the last piece of your friends gum, but what if it doesn't stop there!?
Now now people aren't all bad. Yes, some of us make bad decisions here and there but either we correct them or do something good that cancels out the the evil. Life must have balance and everyone does some small evil now and again but everyone also does some good whether they know it or not.
I'm too lazy to finish but the point is people do good and people do bad. It's all about YOUR perspective. Just stay true you yourself and never give up on what you believe and every thing else will fall into place.
Thanks for reading!