I think there is some value in the principle of de mortuis nil nisi bonum. It's mainly a principle of etiquette, though; in personal relations, if your friend is grieving over a loved one, it's absolutely the wrong moment to enumerate all the reasons why you didn't like the deceased. I think most people who have any manners at all realize this! The question is, should the principle be applied to people writing articles and blog posts about the deaths of public figures, particularly those who were basically famous for being exceptionally evil.
I believe in trying to see the best in everyone. This is a moral principle which can be controversial, but I do believe that each human being has their own unique value. I try to avoid speaking lashon hara, ill speech, about anyone. But that applies even more when a person is alive (and might be hurt by what I say about them) than after their death, so it's not specific to this situation. It is somewhat relevant in how I will decide to comment on the death of a notorious public figure, though.
A more important principle than politeness or even than purity of speech is the principle of historical truth. If a person did terrible things, the fact of their death (which of course will happen sooner or later) should never be used as an excuse to whitewash history or effectively erase the memory of their victims. One should be suspicious of those who try to use de mortuis as an excuse to defend an abuser and exploiter. Stating that Buckley was a disgusting racist is not equivalent to interrupting his funeral to harrangue his grieving family, and rhetoric that tries to equate the two is probably serving a nasty agenda.
However, I don't think it's appropriate to gloat over a death either. Some people love to talk about dancing or pissing on someone's grave, or take pleasure in the pain that they suffered at the end, or even hope they suffer eternal torment. Even if you were personally hurt by someone's actions, that's not a noble way to behave. Obviously, asking for moderation from direct victims is hard, just as asking them to see the humanity of someone who has directly hurt them while their abuser is alive is hard, so there ought to be some compassionate leeway. But for bystanders, I think that taking this kind of joy in someone else's suffering and death morally diminishes the person doing the rejoicing, and doesn't actually do any good in terms of helping the deceased's victims, or even punishing the dead person; they're dead, they don't care.
There's also a logical, even a theological problem with this attitude. Everybody dies eventually; we are not in a position to award immortality to the most deserving. How does it help if the occasional person dies who is really evil enough that we feel comfortable stating that they "deserved" the worst possible fate? You can't treat death as a punishment if it happens to everybody, absolutely without regard to moral standing. Since we all have to die, most people hope to reach a good old age in security and among loved ones, and then to die swiftly without too much suffering. That's about the best outcome you can get, so it's very odd when it suddenly becomes a source of glee if it happens to an evil person. When my neighbour died the other week, I was tempted to find it comforting that she was 82, in very good health up to the last few months, happy, active and so on; she was a good person, and she deserved the very best life available in this imperfect world. But realistically that's just coincidence; Buckley, who spent his life spreading racist and harmful ideas and oppressing those less fortunate, also died at the age of 82 after a successful and (to him, presumably) satisfying life. And all those of my neighbour's peers and family who were brutally murdered by the Nazis before they even reached adulthood were surely far less evil than Buckley. It's a travesty to look for justice in that sort of direction.
I am certainly not saying that everybody should hypocritically pretend to weep and wail over the deaths of hateful people. I have seen journal posts by people whose abusive parents finally died, and of course, being human, they felt relief rather than sadness. Anybody who expects otherwise is badly lacking in empathy. But there's a difference between acknowledging the death of an evil person and the improvement in the average moral standard of the world for not having them in it, and actively gloating and celebrating a death. And I don't understand the justification which says that this person was evil, they gloated over others' deaths during their lifetime, so it's only fair to do the same to them. That doesn't make sense; if gloating over death is a wrong thing to do, which I agree it is, then it's just as wrong to do so in revenge. (Presumably the evil person celebrated the deaths of those they considered to be morally bad in their turn!)
Pain is bad. Mortality is bad. The fact that sometimes they affect despicable people doesn't really improve things very much, in my opinion. And part of the definition of not being evil is that you don't take pleasure in someone else's misery; using the excuse of their past bad behaviour to indulge in fantasizing about such things is morally dangerous.