> I disagree. If you take [action], and it was a necessary and sufficient condition for [consequence],

That's exactly the thing though isn't it. Marijuana is (was) illegal, and has VERY LITTLE amount of violent crime related with its market. Making something illegal isn't enough to induce violence alone. So the [action] of (making drugs illegal) itself alone isn't enough to make [consequence] of (additional violence) to be a certainty.

Or, consider the simple case. I insult you, you punch me. You clearly wouldn't have punched me if I didn't insult you. Who's at fault here?