IMO, hate speech should always be allowed on platforms where users can self-curate their experience. If all the awful stuff gets restricted to one subreddit, then users who don't want to see that kind of content can avoid it entirely. However, if that subreddit gets deleted, all the awful stuff that was previously contained within that subreddit will now start leaking subliminally into posts everywhere else.
It's the same reason why 4chan has 'containment boards'. After 4chan started /pol/, the vast majority of conspiracy theorist and politically radical stuff disappeared from the rest of the website. Furthermore, after 4chan created /mlp/, the vast majority of pony-related garbage went away as well. Having well-defined areas for hate speech and vitriol is a good thing because it effectively keeps it out of everyone else's business.