Jet's un-death at least made for a fun callback in the Ember Island episode.
As for murdering all of the air nomads, it isn't rocket science. The fire bros, knowing well in advance that they were going to murder all the air nomads, pre-positioned troops by all of the air temples. Comet comes, they run in and blow up everything in sight, bing bang boom all the women and children are on fire, the end. Order 66 takes care of the majority of the population, then they spend a few years hunting down and murdering the stragglers.