Mark and Bryon have just rescued M&M from being beaten up by Curly Shepard and his gang. When Mark suggests that they jump a black man, M&M is angry. He points out that they have just rescued him from being beaten up because he was different from Curly and his gang, and now Mark and Bryon want to jump a black man for being different from them. M&M is upset at the hypocrisy they are showing.
M&M gets upset since he just almost got beat up because he was different. Mark and Byron would be beatting up the black guy for the same reason. M&M knows how it feels to be different, he doesn't want to get beat up for it, neither does the black guy.
It would also be racist, btw o.o
M&M is upset because Mark and Ryan just saved him from being jumped because he is different. M&M is angry because they are about to do the same thing when they see the black man. He states they Mark and Ryan are no different then Curly and his gang.