I never understood the idea behind these kind of automatic slur filters.

First of all, why even allow the comment at all? Why not just remove the whole thing? Censoring just the slur doesn't help that much and can just be confusing.

Secondly, by filtering the slur like this, it's less likely that the person posting the slur would be reported and thus no moderator sees the fact that someone posted slurs (unless slur-filtered comments are auto-reported; are they? I don't know). That's the opposite of what you want! If you have slurs that should be disallowed, get people to report those that post them and tell people not to do it or ban them if they keep doing it.

When it's filtered like this and nobody reports the comment, the user posting the slur will probably just continue doing it.

And of course in addition to all the above, there's the problem that it doesn't take into account the context at all, which is what leads to the image of this post.