Lennie was petting the puppy too hard and killed it. He got angry with the puppy, that it died, and he threw it against the wall in the barn.
He was admiring it and petting it because it was soft. Unfortunately, Lennie did not know his own strength and accidentaly killed it. When he realized what he had done, he became angry and threw it against the wall with great frustration.
Lennie's puppy was killed when he petted it too hard and killed it. In his frustration he throws the puppy from him. This is a significant part of the book in that it sets up the foreshadowing for the events that eventually lead up to Lennie's death.