I choose to believe that it's because we have a "soul". I mean, it can't have anything to do with being more intelligent, I'm pretty sure they tested the language of dolphins and found them to be more intelligent than we are.
As to why some humans are good and some humans are bad, I believe that it's because they choose to listen to the animalistic physical form, rather than the aformentioned "soul". If you look at chimpanzees, you'll notice a sort of "mine! not yours!" attitude, and a violent reaction to not getting their way. This is quite prominant amongst humans as well, and it's the soul that helps you ignore these impulses and be a decent person.
And before anyone says anything, I'm of no particular religion and I believe everyone is partialy right. I don't mean to preach, I'm just saying what I think sets us apart. I will never speak of it again. Unless goaded to.