Well, Aang was rising into the air, his eyes and tatoos glowing white. That's not like any bending Azula has seen when fighting against him, so she would probably see it as an Avatar related technique. That said, it'd be more powerful than his normal bending skills, and she would need to do whatever it took to remove that oncoming complication.

While everyone was standing there going, "Oooh, glowy. We're about to get pwned" Azula was figuring that she had precious little time to knock him down before his Avatarness wiped them all out. It's like telegraphing your next punch. Your opponent sees you're gonna throw it, and it's going to be one heckuva punch and could knock them out. Since they see it, and they're smart enough, they throw their own fast technique out before hand. It catches you off guard, and you get knocked back. Happens in videogames too. Big boss battles, mostly. They'll put their arms up and get ready for one big strike that'll wipe you out so you hit them before they can do that. Azula was smart enough to figure out Aang's glowing was telegraphing a butt whooping and she could possibly avoid it by striking him down.

I don't think she's aware of what damage it'll do to his Avatar State, or reincarnation cycle. Let's say she killed Aang but the Avatar spirit is in tact. Azula would know that the next Avatar would be a baby and therefore no threat by the time Sozen's Comet arrives. A baby isn't going to stop them before the war is over. Once it's over, the Avatar doesn't matter. That Avatar baby will be under the control of the Fire Nation, and subjugated to Fire Nation propaganda growing up. Azula isn't thinking that far ahead at the exact second she strikes Aang, but she knows that removing him would keep the Avatar from stopping the Fire Nation.

Azula's practical. She saw Aang's potential at removing her advantage in the situation and judged that striking him with a bolt of lightning could knock him down to the ground. At best, Aang would be dead. At worst, he would be hurt, knocked to the ground, but not unconscious. No matter what, it would interrupt his 'big Avatar move.'

I don't think there was an abundance of planning involved in this particular move (Azula is a planner, no doubt, but never over thinks a situation). She was just removing a potential threat, and possibly removing him permanently.