i think it was probably pretty possible that ozai arranged luten's death, so that he could pick off his father, and iroh would be too distressed after his son's death to want the throne.

but when iroh got so upset that he withdrew from the battle,??ozai thought "well, okay, maybe this will be a little easier than i thought" and decided to just try and convince azulon that iroh was weak and unfit to be leader instead of trying to kill him. but then azulon thought that ozai?? politicking over luten's death and iroh's misery was dispicable, ozai maybe then just??killed him anyway.

of course this still leaves a lot of holes, like what happened to azulon's wish that ozai kill zuko, why was it that ursa disappeared the same night that ozai (supposedly) killed azulon, and how was it azulon's "dying wish that ozai be the next fire-lord, and stuff like that.

I just have a gut feeling that Azulon's death wasn't an accident...but as to who killed him...i don't know...but Azulon's "dying wish" was for Ozai to be Fire Lord...and I don't see how Azulon would say that after saying "you, your punishment(or suffering) has just begun" then when he dies says "you can be fire lord" not believing that for a second...I also think that Ursa's disapperance has something to do with it too...maybe Azulon actually said what Azula told Zuko and his mother found out ,kills Azulon to protect Zuko(remember the whole??turtleduck thing? when you??try to hurt a mother's??kid??they??bite you?)??and flees the palace...but that wouldn't explain??the "his dying wish" mystery