Avatar the last air bender question about zuko?

the part when azula killed aange with the lighting. then she told her father that it was zuko that killed. then when zuko asked azula why she gave im credit. azula told him there was nothing to worry about unless the avatar is still alive. my question is did she know aang was still alive or she was just saying that to make zuko paranoid. Also what was her purpose for telling him this did she plan everything out?