Wow, great answers! So far we have:
[ul]
[li]competition;[/li][li]survival instinct;[/li][li]ignorance;[/li][li]and lack of trust[/li][/ul]
I can see these reasons in many of the problems I’ve personally had getting along. Is there any reason to think that there are better solutions to these problems/challenges than inhumanity? Maybe I’m just being a wuss here, but I think there is value in treating everyone with respect and dignity.
My biggest hurdle is overcoming the animosity I feel towards my ex. He’s mean and self-centered. Even though I sympathize with what perhaps made him mean and self-centered, I cannot like him. There were terrible things said and done. There have been times I’ve even wished him dead, much to my chagrin. However, I’ve tried, for the most part, to be kind to him as another human being even while I was being hurt. I feel that it’s made me a better person because of it. So in my own little world, I’ve overcome the lack of trust and ignorance that caused my inhumanity to another. I don’t know if this makes me a survivor because I will not be beaten down by someone else’s callousness or what.
But if I can do it, and surely there are so many out there who do too, will inhumanity ultimately prevail?
As far as survival of the fittest goes, I can understand that when faced with a battle of sorts, the fitter will usually survive. But fit in what way? Isn’t outwitting your opponent and winning as plausible as beating them down. Okay, if someone attacks you with intent to harm, you either run or fight, right? If you can’t run and talking sense into them doesn’t work, I guess you have to get physical. But I don’t really think the inhumanity here lies in defending yourself, it’s in the attack itself. Why does the attacker have so little value for human life? What makes someone think that human life has less value than a pair of shoes? Or a patch of grass? Or an idea? Or anything else that can be shared and avoid intense hardship on another?
I wonder if aggression can be unlearned by a community as a whole. And how do we do that?
Anyway, I didn’t know if there was going to be any debate arising out of this, but I find it interesting that even in a community like this where bright people come specifically to debate and share information, there still seems to be a quite a bit of intelligent debate that disintegrates into argument, then to spitefulness, name-calling, baiting and all manner of unseemly behavior. It seems that even the best of us cannot always overcome what seems to be inherent in our very nature.
Note to Zagadka: I know there’s been a lot of intense discussion about religion and God, and no doubt you’ve felt as hot under the collar about it as I. But hang in there. You seem to be a gracious person and I hope you can continue maintain a sense of compassion and composure throughout. There are many who struggle together. 
I think that is the most basic desire. We are wired to compete within the tribe for power, and between tribes for resources. As we become a more complex society, we still revert to tribal instincts - basically, us vs. them. This is very evident in racism, but it also appears as regionalism - basically, any competition.