If you come to believe that the time humanity's brain spends occupied with these pursuits leads to negative real world consequences then I don't think the argument you've presented is very compelling. Put another way, if I were someone's therapist and my patient confided in me that, the more time he spends thinking about shooting up his school, the more he feels like actually doing it - I might advise him to spend less time thinking about shooting up the school. If I had the power, I might forbid him from thinking about it at all.
Really, what, but the amusement of edgy people, does humanity gain from entertaining these dark conversations? Perhaps such conversations play a role in the detection or prevention of bad outcomes. For example, police detectives are sometimes taught to identify with the criminal to understand their behavior, and so maybe having access to dark thoughts would help in that regard.
To me, the question of moderation hinges on whether you think permitting speech will be, on net, positive or negative. I don't have data or solid evidence - but I do have intuitions.
