Several people have argued that we should oppose deliberately painful executions because of the risk of slippery slope — even if it's OK to do this to Nazis or people who rape and kill dozens of children, the argument goes, if it's accepted for them it will likely also broaden to other people, both including simple murderers and others who have committed still lesser crimes.

As I've argued in my Mechanisms of the Slippery Slope article, slippery slope effects do often happen — but it's not enough just to say "what about the slippery slope?"; one has to explain the mechanisms that would make the slippage plausible. I point to one such mechanism in my post about arguments against the death penalty, so I think this is something to worry about sometimes. But on balance, I think that this isn't a serious risk here: At least in the U.S., the last two centuries have generally seen a steady movement towards less painful punishments (or at least punishments billed as less painful) and more broadly towards a narrower and narrower set of crimes that can be punished by death.

I'm sure there've been a few moves the other way, but the broad trend is very much in the direction I describe; and it seems to me politically implausible that a decision to allow painful executions for the most heinous of criminals would reverse this trend (though I think such the decision itself would be politically implausible). If I did think there was a serious slippery slope risk here, that would definitely influence my judgment.