Why did Mr. Shhh.....?

My guess is that it was because the guy who was getting beat up by the 3 or 4 (I forget) white guys was an innocent victim being jumped on because of his race.

Also, Mr. Shhh probably figured that the black guy would tell others how he was saved, thus enhancing his own mysterious and terrifying reputation.

PS: was it 1 black guy or 2 getting beat up by the gang????? I forget.

I saw this as his "save the cat" moment (as the guys on Down in Front say). They built him up as a stone cold killer, but also wanted us to see that he was more than just that. A "killer with a heart" so we'd feel conflicted when he died. After all, when you think about it we never saw any reason to actually like Critical Bill, except that he was on the team.