While I disagree with your overal premise, I agree with what you then go on to say. Don't make general criticisms about previous jobs or employers ("it sucked") -- be specific about bad things, how you changed or tried to change or would in future address them, and what you learned.

Saying bad things about previous employers in a constructive manner is not a red flag. (Wasn't there a recent study showing that the people who complain also tend to be productive?) I sure hope so, because I complain a lot.