I wouldn't say that evil is good for us. I think the definition of evil is that it's bad for us. It's the possibility of evil that entails free choice. If we're not able to make mistakes, there is no free will, and mistakes lead naturally to evil. It's possible that in the end, evil is good for us, but that's not a statement I'm willing to make. But evil is simply that; what is bad for us. It comes from our actions, our mistakes, and sometimes from random chance. Illness, death, disaster.