Nobody said that these are all nice people. That's just the way life goes, and so far the show hasn't really asked you to empathize with Kate for torching her daddy. But like Locke has said a few times, none of that matters on the island (or should that be The Island?). They've all been given a chance to start over again, no matter how grave their sins were in their "past lives."
As for AL putting a round through Shannon's center of mass, it would indeed be poor shooting discipline if not for one major factor: the Tailies had been under near-constant attack since they crash landed. Heck, that stewardess got it just minutes (?) earlier. That would make anybody jumpy. Add in some freaky whispers from the jungle and you have a perfect recipe for an ND. Besides, as we've all discussed in various "Zombie Apocolypse" threads, its better to shoot first, shoot again, then ask questions later.