There are lots of examples of irony in Steinbeck's novel, but the most glaring one is the fact that George must kill Lennie because he loves him.
It is a heart-wrenching scene. Lennie thinks, in his naive, child-like way, that he and George are finally going to get the land with the rabbits for him to tend. Instead, George must kill him. He does so because there is no choice. It is either kill Lennie humanely and quickly or watch as the brutal posse tortures him first:
The voices came close now. George raised the gun and listend to the voices.
Lennie begged, "Le's do it now. Le's get that place now."
"Sure Right now. I gotta. We gotta."
And George raised the gun and steadied it, and he brought the muzzle of it close to the back of Lennie's head. The hand shook violently, but his face set and his hand steadied. He pulled the trigger."