In ATLA, Aang is reluctant to murder Ozai because the monks taught him that all life is sacred, and that he must detach himself from the world so his spirit can be free. He contacts Yang-chen, the previous Air Avatar, who tells Aang that he will never be able to do so because he has a duty to the world. In this case, that duty calls for murdering Ozai to protect the other nations from his oppression and tyranny.
This scene enforces that seeking violence outside of self-defense is contradictory to Air Nomad philosophy.
Murdering dictators is the exact thing Aang is trying to avoid to achieve spiritual enlightenment, and Yang-chen doesn't tell him otherwise.
Zaheer is only an airbender for a short while, yet he quickly develops mastery and breaks his earthly tether while having a history of violence, murder, and maintaining spy networks. He wants to change the balance of the earth itself by killing the Avatar and removing all nations. I think they just merged unlocking the Thought Chakra (let go of love) with breaking the tether, but even then it seems fucked that his GF was his only earthly attachment when he is so invested in creating "a new world order" to the point that they were even willing to unleash Vaatu for 10,000 years of darkness.