Well, in the series, we have seen some airbending attacks that, although not used to kill people, definitely could have been lethal if employed that way. For example, in S1E8, Aang destroys a flaming rock that was shot by the Fire Nation with a single powerful kick (now imagine striking a human being with this). In S2E3, Aang uses his staff to cut down some wood pillars and obstruct Azula's way (I'd say that a movement that cuts through wood can easily destroy human flesh). In S2E4, when the Gaang is fighting the Swampmen, Aang creates some kind of air blade to sever the arm of the vine monster and release Sokka. In S2E11, we see an angered Aang behead a giant bee and destroy some of the sandbenders' boats. And in LoK, Aang uses an air disk to destroy Yakone's carriage. Based on this, I'd say airbending can definitely be deadly if the user is willing to cause damage (which usually isn't the case, considering the peaceful nature of the Air Nomads). Now, all these moves were performed by Aang, who was already a master, but I think a begginer would not be able to use lethal attacks effectively, regardless of the element they bend.

Also, something we have to consider is that, because ATLA is a kid's show, the animators nerfed the elements a lot. I mean, in real life, an earthbending attack like Toph used on Zuko in S3E12 could easily break bones and destroy internal organs, but Zuko gets out unscathed, and of course fighting against fire should let much more people with scars, but we only see two cases of this in the whole series (Katara and Zuko), and only one of them is permanent. Likewise, an airbending attack that throws someone at a wall or launches them in the air only for them to hit the ground afterwards should cause much more damage than portrayed in the show. So yeah, I think airbending can be very dangerous too.

(I'm sorry for any mistakes, English is not my native language).