He joined the resistance movement after the X-Men took Big Pig to pieces. Although more likeable than most of the team (which largely comprises villains), he is very bitter about the "death" of Big Pig.
He joined the resistance movement after the X-Men killed Big Pig, a character who was likely seen as a symbol of something positive or important to him. Despite being more likeable than most on the team (who are mostly villains), he is very bitter about the "death" of Big Pig.