In Karak Kadrin there is a very special Slayer. Specifically the King Ungrim Ironfist. Long long ago there was a Dwarf King who undertook the Oath of the Slayer. He did the normal, shaved his hair, spiked the rest, tattoos. The lot of it.
Unfortunately, his oath of kingship came first. He couldn't well abandon his rule and his people. So then the original king passed on this grudge and this Slayer Oath to all his descendants. Ungrim Ironfist is a King first, a Slayer second. He cannot simply throw away his duty for an honorable death.
Now we have Slayers in the Age of Reckoning. Dwarfs join the Oathbearers. From the Armies of WAR page:
Such was the importance of their task; any Dwarf wishing to join the Oathbearers
would be required to swear an oath to the High King himself.
Any Oathbearer active, has sworn their service to the High King. Any Dwarf who shamed themselves while in said service could become a Slayer. In this way you can argue for Slayers for the game. They fight, recklessly as they would, but accept aid from the others because they've not yet fulfilled the oath. To see the Doomstrikers forged and the armies ranged against all Dwarfs fought.
The best (or worst) Slayers grow in power. They can't just commit suicide and be freed of the shame. They have to always improve in skill and power. They are supposed to seek the toughest foes and greatest evils to fight an epic battle. One that will destroy them and wipe away the shame.
You know... that or Mythic can just say they've popped over from the Slayer keep.
"'Ere lads, go smack up those armies. Live long enough and you'll get a fine axe and we'll go invade the Inevitable City"