What do you think are the reasons that humanity has largely started to treat their prisoners and foes in ways that do not involve horrors such as torture?

Our current relationship to a concept like torture comes out of dualism. It's not cruel to put someone decades into prison for a nonviolent drug offense but it is cruel to inflict physical pain because that violates the sanctity of the body.

It's also bad to practice euthanasia for terminally ill patients that are in a lot of pain and suffer a lot.

Depending on what memes are around when civilization get's rebuild it not clear that we will get the same position and the same social consensus will arise. It also possible that over time the use of capital punishment of a country like Singapore will spread even without straight collapse.