At the beginning of Thor: Ragnarok Surtur tells Thor, "I am Asgard's doom and so are you". Isn't Thor the good guy? Then why does Surtur say this?
Surtur tells him that because
he'd be the one that will initiate the Ragnarok.
Near the end, you can see that Thor wasn't able to kill Hela, so, in order to do that, he asks Loki to put the Surtur's crown in the eternal flame, so Surtur can destroy Asgard along with Hela.
Thor: Asgard's not a place, it's a people. This was never about stopping Ragnarok...it was about causing Ragnarok.
Thor: (to Loki:) Go to the vault. Surtur's crown. It's the only way.