Yes. The problem with CO poisoning (or, Nitrogen) is that it is painless, and people actively resist that. Some people want it to be about retribution rather than justice.
And that, I think, is one of the best arguments one can make against the death penalty: it’s about revenge. The point of having a judicial process and the rule of law is (also) to prevent the aggrieved from acting out as their emotions dictate, because that’s generally against the interests of broader society (and amongst other things is how blood feuds start).
Interestingly enough, that's also a strong argument for the death penalty. We cede the right of private retribution and the state assumes that responsibility, carrying it out in a more structured and ostensibly fair way than mob justice. Historically, the state assuming the responsibility of giving citizens justice (including for capital crimes) was one of the key turning points in the rise of centralized rule and in Europe, the transition from the Middle Ages to early modern Europe. A great book that explores this topic is, 'The Faithful Executioner,' about a 16th century executioner in what is now Germany. I imagine people today may disagree about how important that still is.