Idiots make up crap about all kinds of things, not just violence or other illegal acts. Ideas outside societal norms often attract bad PR. If your primary goal here is to improve PR, you would have to censor posts by explicit PR criteria. The proposed criteria of discussion of violence or law-breaking is not optimized for this goal. So, what is it you really want?

Discussion of violence is something that (you claim) has no positive value, even ignoring PR. So it's easy to decide to censor it. But have you really considered what else to censor according to your goals? Violence clearly came due to the now deleted post; it was an available example. But you shouldn't just react to it and ignore other things, if your goal is not to prevent discussion of violence or crime in itself.