I don't know who "we" is in your comment, but the question of whether capital punishment exists or not is a totally different question from whether the rule of law exists or not. It's perfectly self-consistent to support the rule of law while believing that some of the rules are wrong (e.g. that the state shouldn't kill people as a punishment). In fact that's essential to the whole thing working, since there's no way everyone will agree on the best set of rules for society.