You will not change them, and everyone present already made up their mind on their behavior.
Speaking only positively about people distorts the reality.
I'd say calling him out as a racist is not exactly speaking ill of the dead in this case.
When someone I know dies, I speak frankly about them, good or bad, because to do otherwise is a lie, and the most disrespectful thing to do is to misrepresent a person who no longer can represent themselves.
Scott Adams did what he did, that's surely not in question. Honor his life by speaking frankly about how he affected oneself and others, good or bad. Let the chips fall where they may.
Reputation guides your behavior toward that person. But they're no longer around. There is no behavior toward them. They're gone. Their reputation is no longer relevant.
It also culturally informs someone's perceived suitability as a role model. It doesn't matter to the dead person if they are held in high or low esteem, but it may matter to people in their formative stages deciding whose influence they follow and whose they shun.
> Why is their reputation relevant? They're dead.
I agree with your conclusion, but not with your premise.
We can't "do" anything for the dead. They're dead. What's more, since they're dead they don't care what we do or say because they're, you know, dead.
Anything we might do or say in reference to dead folks is for the benefit of the living and has nothing to do with the dead.
That said, you're absolutely right. We should remember folks for who they were -- warts and all -- to give the living perspective both on the dead and the dead past.