We see George reveal his compassion to Lennie when he speaks with Slim about their circumstances in chapter 3. From that point on out, George remains in defensive mode for Lennie. George cares about Lennie and cares like family. In families, it's okay for members of the family to pick on each other, but, and most importantly, others on the outside are not allowed to pick on family members. George remains in a defensive mode from the time they arrive on the ranch until the end. From there on out, George is protecting Lennie by dictating what Lennie can and cannot do and say. When push comes to shove at the end, George prefers to ensure that Lennie's death is comfortable as opposed to torturous. When George moves into this defensive mode, I think his entire capacity to tease Lennie leaves him.
In Of Mice and Men, George explains to Slim, in one of their close conversations, that the reasaon he quit teasing Lennie a long time ago was that he almost killed himself following one of George's pranks. George explains how he and Lennie and some other young men had been horsing around at the river and George had told Lennie to jump in, and Lennie, being as devoted to George as he always has been, jumped in. George did not realize that Lennie could not swim! LEnnie damn neardrowned before George could get himout. That s why George quit teasing Lennie.