Yes, in the wild they will eat snakes, lizards, birds, etc. In captivity a little white chicken meat (no bones) is a good source of protein as is mealworms.
Some monkeys do eat (or primates better said) chimpanzees are even caught on film ripping limbs from their own kind, and will lead to death. They'll eat the carcass and so that answers ur q.
Yes, chimpanzees gather in groups and hunt in an excited frenzy, howling, chasing rounding up, and eating smaller monkeys alive. I saw it with my own eyes on a docco', it was really quite violent.