The Mercykillers are one of the Factions that contest for the minds & souls of Sigilites in the Planescape setting for Dungeons & Dragons. Their belief is a simple one: Justice is the most important thing in the multiverse, and all dedication should be to serving its cause, pure and free of all distractions and dilutions, such as mercy - hence their name. Needless to say, the Factions had some rather Ethically Stupid members in their ranks, but the Mercykillers manage to take Lawful Stupid to a whole other level above and beyond their allies the Harmonium and the Fraternity of Order.
Seriously, as a Mercykiller PC, you're not even allowed to fight save to defend yourself on your own; if you see someone commit something wrong, you need to make your way to an authority figure, tell them about it, and ask permission to punish the crime. Expect stiff ability penalties if you break this law, up to and including being cut down by your fellow Mercykillers as a traitor.
The Mercykillers were born out of the historical "Great Uprising", when the then-49 strong Factions were told by the Lady of Pain that they had best cut themselves down to 15 factions within the next fortnight, or she'd do it for them, on account of the civil war they had going on in the streets. In that chaos, two small justice-worshipping factions decided they could make it work by sticking together; the Sons of Mercy, a Lawful Good faction who tried to exploit the justice system in order to help those who were wrongly imprisoned or facing too-severe crimes, and the Sodkillers, a Lawful Evil faction of thugs who sold their services as legbreakers in the cause of revenge to anyone with the jink. Somehow, the result was a fundamentally Lawful Batshit Insane faction who went on to take complete control over the prison system of Sigil - no, it doesn't make a lot of sense, but this is Planescape; what does?
The Mercykiller creed is summarized as the Eight Tenets of Justice:
- I: I will uphold Justice before all else, purging the multiverse of those who break the law.
- II: In all situations, I shall weigh the rights and wrongs with a fair and impartial mind.
- III: I shall decide where Justice must fall under the law, and I will mete out that Justice with a firm and unyielding hand.
- IV: I believe in the righteousness of my Faction; we alone answer to the higher law of Justice.
- V: I will not pass judgement on good or evil, only on law-abiding and law-breaking, for therein lies wrongdoing.
- VI: I will punish the guilty as the crime demands.
- VII: I will be diligent in my pursuit of the guilty, and while so engaged I will remain innocent of any wrongdoing in the eyes of others.
- VIII: I will never release a lawbreaker until his sentence is carried out.
As you might expect, the Mercykillers are full of fundamentalist crazies; prior to the Faction War, the Mercykiller Factor wants to arrest and execute the Lady of Pain. That gives you an idea of the quality of nuts in this particular fruitcake. After the Faction War, the Mercykillers broke up again into the Sons of Mercy and the Sodkillers.