So... since Illaoi considers Nautilus a rival, does that mean our big Titan is undead?

Or the unwitting pawn of an even greater power. Thresh's unique taunt when he hooks Nautilus is an interesting relationship hint that's never been expanded on. We could go either way, really.

Potentially, potentially not. There are many Buhrun gods aside from Nagakabouros, and some may not be so nice. While I can see the connections for Nautilus and the Mist being very clear as opposed to other theories, I wouldn't throw out the idea that Nautilus was corrupted by something in the deep sea still, and that by Illaoi's beliefs, that power comes from some god that's not viewed all that positively on Buhru.

She probably lumps him with the ascended.

He is a being who has been transformed into a potentially immortal form, but in her eyes, cut off from the flow of the universe.

I dont think so. Maybe the day that transformed into what he is now, he found the bearded lady and this tried to claim him, but his spirit stay trapped in his diving-swit. So now he hates alll the deep sea creatures , including nagakabouros. And considers him her enemy because he despises her god.

Isn't being "weighted down" an important part of Nautilus theme?

Maybe that is the reason she sees him as a rival.

I don't know what he is, but I don't think he qualifies as undead.

is not undead, he was dragged into the deep by a dark force and had his flesh morphed into something inhuman and merged with the suit. He also fought of the mist of the shadowisles alone when it tried to take bilgewater aswell which is not very undead like as all undead are tied to the shadow isles.

Maybe the darkness that claimed is something that nagakabouros conciders a corrupting force which makes his natural enemy.

But the most likely reason is that simply can't let go of his past and thus he can't move on making him a living embodyment against everything stands for. Also that he seem to hate all creatures of the deep aswell which probably meansnagakabouros, for all we know may have personly met nagakabouros.