Quite a few (not all, but the more famous ones) write manifestoes in which they describe how they were radicalized, cite other manifestoes and mainstream racist commentators who inspired them, along with Adolf Hitler, and make it very clear what their motives are.

> youth bullying and ostracism

... which itself counts as free speech?

A lot of people get bullied in a lot of countries. Few of them turn to mass murder. The mass shooter phenomenon is mostly America, plus a few people who've been radicalized by the same racist discourse on the same message boards (Anders Brevik, NZ mosque shooter).

In quite a lot of cases closer investigation suggests that they were ostracized precisely because they'd already chosen hate.

Free speech doesn't protect against libel/slander. It also doesn't protect against speech that induces violence or panic (the old "yell fire in a theater") And I argue impersonating celebrity voices to radicalize others falls under the latter.

So yeah, I would call this "wrongspeech" and wrongspeech is already banned.