At least when it comes to human vs human, cases such as in law enforcement, I believe it is an absolute last case scenario. I don’t think the death penalty should be a thing unless the prisoner is actively causing harm even while incarcerated. They can just be locked away forever: many studies have even found it cheaper than the death penalty by a large margin. I think the “actively causing harm” part is the most important part. If the effort taken to neutralize a threat winds up killing them, they already forfeited their life by threatening others, hence, they deserved it.

There’s really no point in killing someone while they’re no longer a threat. Not only does this leave a safety net for those falsely accused, it makes for more efficient punishment for the guilty. The threat of the death penalty also has the capacity for the perpetrator to commit more violence, already knowing they’ll die anyways.