The 7 Commandments of Noah are for the seventy nations. However, right now there is no beis din and we are in galus, where the law of the land is the law.
So, if NY had a death penalty and Levi Aron’s crime met the criteria (it does, unless he’s insane), then I would have no problem with his being executed.
He who is merciful unto the cruel will one day be cruel unto the merciful.
This, of course, is a hypothetical question – because NY, being the bastion of liberalism that it is, does not have a death penalty.