No, there are lots of people who have one or two crackpot opinions who aren't crackpots.
I think you misunderstand the expression. When there's a crack in the pot, the whole pot is cracked. It doesn't necessarily mean the pot is completely useless but you probably shouldn't trust it with anything valuable.
Actually, Marxism promotes self-defence by the workers against the violence of the ruling class. You remember self-defence, don't you? That thing that you approve of and I don't?