I think you both might be wrong.
I have more questions than answers, I'm afraid.
Our brain is more developed, but is it significantly more developed and is that difference even relevant? We can observe other species and judge their mental abilities, but who judges ours? Again, it's a matter of perspective, but all of our actions stem from our own instincts. Our tendency to kill "for fun" or to inflict pain comes from some psychological need, that is as convincing as any hunger or need to reproduce. Even the question if we even have free will comes into the picture if you want to judge our actions.
Dogs kill for fun. Cats kill for fun. Even dolphins, supposedly the second or third most intelligent animals, kill for fun. I don't think our species does anything that other species don't, I just think that we are slightly more efficient. In fact, the closer we come to our own species, the more of our own nastiest tendencies we see.
The thought that our species is flawed is not something I can agree with, either.