The state-administered judicial violence of locking someone in a cage until they die of old age strikes me as pretty similar to the state-administered judicial violence of executing someone, in terms of severity.
I don’t love the way American prisons work either, but I guess we have to agree to disagree on that particular point.
The point (or one of the points) is that execution is reversible. I mean there are some cases of people who sat in prison for decades when it was determined they were innocent. If you accept potential failure of the legal system (which is obviously the case) how can you be a proponent of executions?