A monstrous killer terrorizes a peaceful homestead. From far off, a force comes that is both wild and civilized--a killer, like the monster, but a killer on the side of the peaceful and civilized. When the monster next attacks, the monster-killer is waiting and beats the monster, who retreats back into the wilderness.
The monster-killer eventually follows the monster, probably because a bigger threat comes to the peaceful homestead. The monster-killer fights his way into the monster's fortress and kills the monster.
At the end, the monster-killer rides off, either because he cannot live a peaceful life with the homestead or to face another monster, though this one may kill him.Now, if you said that sounds like Beowulf, I'd agree. And if you said that sounded like an archetypal Western, where a gunslinger comes to a peaceful community to defeat the other, more monstrous gunslingers, I'd agree with you too. There's some significant overlap in the classic fantasy of monster-killing and the general form of the Western.
Now, this gets complicated because Westerns and Fantasies have some significant wiggle room to their genre. And Beowulf, while being classic, is hardly in the mainstream in its final "killing off of the hero": the classic Western may tend towards melancholy ("I've made this town safe for law-abiding citizens, but I'd never fit in because I'm a dangerous man at heart, oh woe is me!"), but the classic Fantasy Quest tends towards kick-ass-dom ("I've made this town safe from orcs, I'm gonna go kill a dragon, fuck yeah!").
This bears more thinking about, especially when I'm not at my parents' house for Passover.
(But to cite my inspiration: In a weird confluence of blog posts, I read this LitReactor article on the weird western and this Amazing Stories post on the archetype of the Western hero in speculative fiction. So is this a thing now?)