I think he might've wanted to break the cycle. So that there would be no more Avatar.

Umm, yep. See, the Fire Lord thought that uniting everyone would make them able to do more things, or would spread the peace of the Fire Nation outwards. He was...not right, about that, and somehow he got everyone in his nation to agree to doing it, anyway.

He was! He was caught in a storm, and fell into the ocean, and the water froze him and Appa - his sky bison - and a century later my grangran found him and cracked the ice open and then they went and saved the world!