I've seen a lot of kids my age become Communists or Nazis due to internet-induced radicalization; I was rather close to becoming a radical leftist myself. Personally, I think it's harmless; they tend not to hold these views for a long time when tempered with the saner views of their peers and broader society. It's like the chicken pox - you get it once, become a bit nasty for a while, and then you're immune to further insanity.

It's when the person in question is mentally ill or socially isolated (i.e a NEET living in their mother's basement) that radicalization becomes harmful, because there's nothing to bring them back to the baseline. Marginal identities attract marginal people.

(I have a theory that online transgender-ism is spreading in exactly the same way - note the massive amount of trans people online who are also NEETs, weebs, and/or communists. But that's a story for another day.)