Can I kill everybody?
Solution 1:
There are definitely characters you can't kill, or that you can't kill until certain points in the plot. I'm not sure about all of them, but Paul springs to mind in the first mission. I've unloaded a great deal of ammo into him many a time, and he just keeps running around and eventually shoots me to death with a plasma rifle.
Your best bet is to save before you try to kill any main-plot characters, I suppose; hopefully that won't ruin the experience for ya.