MasutaKokoBot
well, he might have realized he was going against the avatar, but he was trying to get her while she was still young, but he was stopped before and imprisoned. when he was free, he he still had a chance to finish what he started, as korra is still young. now that he became an airbender, it could have increased his chances of success and such
so i can see why he would think he could defeat her (and he almost did, if it werent for the other airbenders)
c:
I think his first kidnapping attempt was more for brainwashing and less for killing. I don't think they were above killing her in the least, but I figured they wanted the avatar following their ideals and to be a weapon for them. Also, if they did kill her Raava would just go on to the next vessel.
And don't get me wrong, I love Korra, but I think it was more about the avatar spirit and not her. Now I'm not sure if the Red Lotus knew about how Raava was weakened and the previous avatars were eliminated, and if they did know it would make a lot more sense. But it's still a centuries old spirit vs. a ragtag (though highly powered) group of misfits.
Plus, the airbenders helping Korra brings up another point: did anyone at some point remember that Korra's allies even had allies? If it wasn't for the Krew breaking the airbenders out, the Red Lotus might have succeeded. They probably needed more people guarding the the air nomads~