I found the guy who has it, but when I kill him I get the whole threads of fate severed message. Already completed the mainquest, is he important for some reason, or can I just ignore it?

That's that Bosmer dude who's a false hero or something eh? If so, ignore it. He's no one.

(1) Crazy Batou seems to be a relic from preceding versions of Morrowind, I don't know. He isn't important anyhow, the less he is a game critical being. The disaster message is completely false.
You can kill him and get the helm whenever you want during the game, without spoiling or losing anything.

(2) The bosmer dude belongs to anoter helm (the best one in the game). Check daedric quests.

Ah Crazy Batou. That's right. He was going to be part of the main quest until they decided to axe him for one reason or another.