I am pretty sure proponents of the death penalty will change their mind when the concept of the death curse is explained to them, and that it could be aimed at them unless other practitioners are involved in the process. Having all the guards in a prison drop dead because an improperly handled Warlock would do that.
It's not even really about the death penalty vs imprisonment. It's about arresting and going through the legal process, vs. shooting from surprise Kincaid-style. Without magical help, I think the latter is the only safe way for mortals to deal with strong (sorcerer- or wizard-level) warlocks.
(Even Kravos, who was neither very strong nor very skilled, managed to create the Nightmare in prison.)
But that's not going to be acceptable (for very good reasons!) to most nations.
IE some level of help from magical types will probably be necessary.
There must be a way around that when it comes to execution, otherwise warlocks wouldn't get the chop.
I don't think that there's anything 100%, but I also think it's not often a problem.
Death curses seem to require Council-level power or close to it, and Council-level talents are super-rare (one in a million). Your typical Sells or Kravos or Aristedes type warlock isn't going to be capable of one.
And the strong warlocks may be too crazy to try (e.g. Grevane). It seems to require knowing & accepting that one is going to die.
Someone like Morgan (who usually did executions) might be strong enough to shield against a death curse from a relatively weak wizard, and/or possibly the spell-breaking property of Warden swords can work against them.
Takes vision. Hence the bagging 9f the head.
No... Maggie Sr aimed her death curse at Lord Raith who was definitely not present. (And Harry's description of preparing to use his against Grevane when he thought Morgan was going to kill him in DB also sounds like you can just aim it mentally.)
Vision might matter for less capable spellcasters, but it's by no means absolutely necessary.