I've had a rather unsettling feeling that Azula has been scheming to take the throne of the Fire Nation ever since she was a child (you can see she's set on it during Zuko Alone). I've even come up with a rather nasty theory that she has been taking steps to bring herself closer to such a goal. I'll post it here. (WARNING: contains massive spoilers!)
I'm led to believe that Azula has been systematically eliminating members of the royal family who stand in her way. First, she took out Fire Lord Azulon, and judging from the circumstances, I think she was also planning on pinning the murder on Zuko to take him out of the picture (she did have Zuko's knife, after all). Unfortunately, Ursa catches on, and takes the blame for Azulon's murder instead of Zuko (what happened to Ursa is up to debate). After Azulon's death, Azula rigs it so that Ozai becomes the next Fire Lord instead of Iroh, so she'd have a chance at the throne. Unfortunately, Zuko would still be in her way, since he was firstborn (despite Ozai detesting him) and rightfully next in line for the throne.
I'll bet Azula was waiting for a good moment to take Zuko out of the picture, but had her problem taken care of (mostly) by Ozai during the Agni Kai. With Zuko banished and denied his birthright, the only person who would be left in Azula's way would be Iroh. If Ozai were to die and Iroh were still alive, he'd be next in line for the throne. If Iroh were dead, however, Azula would be next in line for the throne.
And now that Iroh is either dying or dead, Azula has no more competition for the throne (since Zuko has no right to the throne as an exile). The only person she'd have to eliminate now is Ozai. Of course, she'll have to wait for the right moment to strike, so as not to cast suspicion upon herself.
There, how does that sound? Anyone wish to debate on this?