During Legend of Korra Season 3, Zaheer is pretty amicable in almost all situations and very rarely hurts anyone he doesn't have to; in fact he admonishes Ming Hua for attacking a lowly guard. Aside from keeping the conflict going (as the plot necessitates), why wouldn't he free the fledgling airbenders? In fact, he really only lets go of the single airbender who is really a threat to him (Tenzin).
Is there any reason for doing this other than plot movement?
My guess is: because he still needs hostages.
After the avatar surrenders and all the airbenders are freed, there would be nothing to stop the other from trying to free Korra. However, still keeping the airbenders hostage would give them some pause: if they rush in, all the airbenders would get killed and Korra just sacreficed herself to save the airbenders.
Also, they tried to kill the ones who came to pickup the airbenders, in order to get rid of people who could stop them.
I would argue that he knew he "had to" keep the airbenders subdued and hidden away. Zaheer's combat advantage was basically that he was an airbender, and most benders/fighters that would come up against him have very little experience dealing with an airbender. Up against an airbending master like Tenzin, his airbending fell well short. Also, airbenders are incredibly mobile, being able to fly great distances and do this quickly. Zaheer needed to capture Korra, and needed time for the poison to do it's work in order for them to murder her while she was in the Avatar state. This would be easily foiled by a group of airbenders who could fly off rapidly to get reinforcements, and fly around scouting for where they were hiding with Korra.
In the end, not only does Jinora, a soon-to-be master airbender, use her powers to find where Korra is, she also used her knowledge of obscure airbender techniques to subdue Zaheer. So it's pretty clear why Zaheer wanted the airbenders out of the action when you see who played such a pivotal role in defeating him.
Because Aangs dream was to rebuild the air nation and tenzin wanted to make that happen and Zaheer knew that from the get go. So after he killed Korra he would watch tenzin suffer an well enjoy it cause that who Zaheer is. Only he failed. BIG TIME!!
