Ernst Zundel, author of "The Hitler We Loved and Why," whose second wife said "By the end, I thought he was evil incarnate," is a political prisoner in Germany, currently going through a criminal trial because he denied the Holocaust. He was also deported from the US and Canada, bringing our nations into the mess.
I see a parallel to capital punishment, because like the majority of death row inmates, he doesn't deserve what's happening to him but he's also truly awful. It makes it hard to want to lift a finger to help him, especially after reading lovely quotes from his Hitler worship. But like any other type of repression, Germany's habit of imprisoning people for spreading hate has consequences. People who buy the 9/11 conspiracy theories are looking at Israeli/Jewish connections, and it's easy to tie actual repression of anti-Jewish sentiments as a way to support their theory that the truth about 9/11 is being repressed.
Anyway, time to assign blame: as for the US, we deported him for a visa overstay. I think it's possible that would've been overlooked for a non-controversial person, but I don't know how the enforcement machinery operates. I can't confirm that we screwed up, just suspect that we did. Kind of an interesting parallel to another bastard, Ward Churchill.
Canada refused asylum and deported him for potential involvement in actual hate crimes, but used secret evidence. Blame Canada on that - you couldn't (easily) get away with that in an actual criminal trial. He was also previously convicted for the violation of spreading hate on the internet - in other words, thought crime.
Germany deserves the most blame (after Zundel) because of their stupid laws. Their democracy is a crock if it can't withstand a bunch of fools telling lies 50 years after Hitler got a small taste of what he deserved.
And a special raspberry for Amnesty International - here's a stupid letter they wrote in 2003, apparently buying into the idea of a hate crime where the only "crime" was hate (note it's reproduced by a Zundel supporter, though, so some caution about what AI actually wrote). I tried to find what AI thinks about German Holocaust Denial laws in general, but didn't find anything.
UPDATE: edited for politeness.