He doesn't trust people because he knows they are all the same. Everyone cares about their own survival and nothing else, just like him. Since he is more than willing to kill for it, so are they. After all, he has endured through all these years, leading him to be alone, it was the only conclusion that made sense.
Sure, you can break a man. Bend his will, even, but be careful with the ones that break easily. Those are the ones you have to keep a close eye on. Those are the ones that play possum and hide in the shadows. Just waiting for their time to strike! That's when you're most vulnerable. When you're surrounded by friends.