Dwarven Troll Slayers in Warhammer are dwarfs that feel they are so irrereemably dishonoured, that they go in search of one of these by finding the biggest and scariest monster they can find, (most often trolls, hence the name), and fighting it to the death. Most succeed at this, but a few screw it up by winning, and are forced to find something even bigger and nastier to fight. There are a few Slayers that suck so hard at it that they're among the toughest and most skilled fighters in the game. The Prestige Class for surviving Troll Slayers is Giant Slayers.
Part of the slayer oath is atonement by destroying threats to the Dwarf race, not just by getting killed. If death were the point, they could just jump off a cliff. They have to take as many evil monsters with them as they can.
Also, the Oath is that they must die 1) In battle and 2) In a fair fight, meaning that they can't throw the fight.
There are even Prestige Class' for those that survive being a Giant Slayer- Dragon Slayer and Daemon Slayer. Consider that in this setting "killing" a daemon just banishes it, and it seems unfair that the only Slayer since (the god) Grimnir who may have actually killed a Greater Daemon doesn't merit a mention on this page. He's still alive... He ends up replacing Grimnir as he is to badass to die.
Warhammer 40,000 has no lack of these, perhaps the most obvious goes collectively to the entire the Astral Knights chapter of the Space Marines, an otherwise unknown and unmentioned chapter. The Imperial forces fought the World Engine, a planet-sized Necron spacecraft that had annihilated 1/3 of a battlefleet without anything they did as much as breaching its shields - laser batteries, boarding crafts, teleport beams, you name it. The Astral Knights' Chapter Master eventually decided on trying to reach it by ramming through its shields with his battle barge, the flagship of the chapter's fleet. That finally got them close, and he and every available marine got in the drop pods, went to the surface to the ship, and for 100 hours fought against tens of thousands of Necron warriors, wrecking any control node, generator, or any possibly important piece of scenery they came across. Finally, when the space marines were whittled down to the Master himself and five other marines, they detonated thermal charges which brought down a massive tomb complex containing many of the Necron command arrays. That finally brought the World Engine's shields down, allowing the fleet to take it down with cyclonic torpedoes. Lots of cyclonic torpedoes.
The last stand of Ultramar is another awesome one. 100 Ultramarines pitted against a full Tyranid hive fleet in 24 hours of solid, bloody combat. When the last Ultramarine died, just as re-enforcements arrived, the ground of the Ultramarines fortress monastery was that thick with Tyranid corpses that almost the entire complex was buried. Damn, say what you will about Ward's love of the Ultramarines, that is badass.
And rather more interesting than just a simple Guardsman...
And then there's the Ork Warlord Tuska, whose Waaaaagh came to a bloody end on a Khornate Daemon world in the Eye of Terror. When a Daemon Prince crushed his Weirdboyz with a single gesture, Tuska used the last of his strength to reach up between the Daemon's legs with his Power Klaw and made a gesture of his own. Khorne was so impressed by this that he resurrected the Orks and moved them to his own realm to die fighting over and over into eternity, which is a Warrior Heaven to them.
Though this is something of a subversion, as Old Man Henderson wound up being reincarnated after personally speaking with the entity he had slain. Which is a Crowning Moment of Awesome in and of itself, as Hastur is to a human what a human is to a dust mite, if that.
Dungeons and Dragons has had these built in since its inception, but special mention goes to Oriental Adventures for having a spell that allows one to gain a ridiculous combat buff - the only downside being that when the spell runs out, the user in question disintegrates, making them extremely difficult, if not impossible, to resurrect.