The Buddha said something to the extent of "evil exists so that good can show its superiority." I don't think that's the exact quote, but it's something like that.
Christianity only works for some people. When I was a Christian, I had no way to deal with my issues of vices that I had. It wasn't until I became a Buddhist, and started practicing Buddhism, that I began to be a better person. Granted, I still have a long way to go. But humans, by nature, are imperfect, I believe, in order to have a goal to work toward, which is transcendence. Christianity never really offered that to me. All it did was teach me that I was to remain in my vices, and that all I could do was rely on god for forgiveness, and hope that he accepted. In Buddhism, I'm actually taught that I can change for the better, and given the tools necessary to effect such a change.