To go from a brilliant satirist to becoming terminally online and just completely falling off the far right cliffs of insanity is incredibly sad. And unfortunately, this is plight is not uncommon. It is incredibly dangerous to make politics part of your identity and then just absolutely bathe yourself in a political media echo chamber.
If two or three days ago, not knowing he was sick (which I didn't), I had said to someone "That Dilbert guy seems to be sort of a whack job," why would it matter that he was alive to hypothetically defend himself? It's extremely unlikely that he would ever be aware of my comment at all. So why does it matter that he's alive?
But it's basically getting the last word in because the other party is unable to respond. It's seen as a little uncouth.
On reddit, it's kind of like those people who respond, then block you to make sure you can't respond. They aren't there to make an argument or convince you, they just want to get the last word and they're doing it in a way where you cannot respond.
Like I said, I don't entirely agree with "don't speak ill of the dead". Especially for figures who used their platform to elicit responses. But that's one of the reasons behind the sentiment. Right, wrong, that's for you to decide.