And there are plenty of people willing to step in for Scott and defend him, as evidenced by the contents here.
Someone dying doesn't mean the consequences of their words and actions disappear and acting like we should pretend that death washes away those consequences is silly.
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.