Why is he angry at the dead puppy?

Answer 1

In Of Mice and Men Lennie gets angry at the dead puppy sheerly because it died when he pet it a little too hard. He thinks that killing the puppy will make George mad, and therefore not be allowed to tend the rabbits on the farm he wants to run with George.

Answer 2

It is likely that he is angry about the situation that led to the puppy's death, rather than being angry at the puppy itself. He may be upset because he feels responsible, sad for the loss, or frustrated that he couldn't prevent it.