In the first age, in the first battle, when the shadows first lengthened, one stood. Burned by the embers of Armageddon, his soul blistered by the fires of Hell and tainted beyond ascension, he chose the path of perpetual torment. In his ravenous hatred he found no peace; and with boiling blood he scoured the Umbral Plains seeking vengeance against the dark lords who had wronged him. He wore the crown of the Night Sentinels, and those that tasted the bite of his sword named him... the Doom Slayer. -- Anon Guest
There were other names. The Soul of Vengeance, The Terror, The Unstoppable Force. They go on a bit. Those lucky enough to survive his Wrath spread tales of what he did to the enemy. Word got around. Those he did not kill were those who did not catch his ire. Those who caught his ire... did not live to tell the tale.
First, he came for evils that were wont to destroy the world. Those who merely wanted to rule it. Those who wished to rule with an iron fist, however, gained the Slayer's attention. There was always another evil. There was, after all, a lot of world and only one Slayer. For that, many villains were grateful.
If you are literally the worst? They say the Slayer will come for you. Those caught in the edges can't decide if they are good or bad, but they stop the greater evil and that has to be good? Yet they don't seem to care about the wreckage they make along the way. That has to be evil. Nobody had ever been close enough to the Slayer for long enough to ask them about it. Not until the third age, when war was waged more with paper and fiduciary backstabbing than the type with knives.