why would one refuse to kill a mythic dawn supporter and descendant?
I wouldn't call him a mythic dawn supporter.
He's just a boy.
A naive fan boy who has no idea what he's getting himself into.
If you let him live, I doubt he'll ever consort with Mehrune Dagon ever again, as he hints when he comes back to Dawnstar.
Plus, hearing his frightened screams as he hurtles down the stairs after witnessing the bloodthirsty dremora wielding huge 2 handed axes is worth it!