Humans; good or evil by nature?

There is a rather large debate surrounding the issue of whether humans are good or evil by nature.

Personally, I believe that humans are self-serving by nature; not necessarily evil, but not with only pure intentions either. I am of the opinion that by default, humans will seek to make decisions that will best benefit themselves. However, I don't think that means that they will never help another human out, only that when it comes right down to the line, they will mostly choose what is best for them.