The Mercykillers believe that justice is everything, and no one should escape justice. Those that try willll have the Mercykillers on their tail, so the smart thing is just don't try. It's the whole reason laws exist - to see that justice is carried out. Justice purges the evil in folks and makes them better, fit to belong in the multiverse. Once everybody's been cleansed, then the multiverse reaches perfection, and perfection's the goal of the multiverse.
Justice is absolute and perfect, but it's got to be correctly applied. One must separate the criminals from the innocent, so he doesn't make a mistake. Mercy's an excuse created by the weak and criminal. They think they can rob or kill and then escape their crimes by pleading for mercy. The Mercykillers are not so weak. Every crime must be punished according to the law. There are no such things as " extenuating circumstances."
Some smart cutters figure they'll hang the Mercykillers by their own yardarm and accuse them of some of their own crimes. Well, the Red Death'll just smile and say it answers to a higher law. Charged with protecting justice, they can do things others can't - all in the name of justice. How else could they survive their own ideals? 'Course now, other folks don't agree with Red Death logic. To them, the Mercykillers ain't above the laws or even right in what they do. The Mercykillers don't like such folk's attitudes, but they can't hang a being for its opinions - at least not in most places. See, the Mercykillers say they don't make the laws, they only enforce them. All in all, they're no better than the rest, but no worse than a few.