In 3/4 endings, all of which being endings you're much more likely to get first before the fourth one, a dog/animal that is visibly a dog dies, though who kills it and whether or not it stays dead varies.
Possibly implied in the secret files. A character also uses his children as weapons, although it's played for humor and the children don't seem to be hurt.
A character named Grin watches another character without her knowledge, and this is not condemned by the narrator. An optional quest is helping him deliver a message to her.
To add to the other comment - the boss does not actually die, you "befriend" it and it disappears. It is visibly injured, though, and the game talks about killing it.
YES. There is a character in a cage that specifically mentions that he's a cannibal. Also, you can put the body of a person into a cauldron, though the body is not consumed due to this.