I know the lurker's default fatality 'The Maw' is where it grabs the marine by the head, lifts him up and head-bites him with it's inner jaws. I am pretty sure the soldier's default 'Hole Punch' fatality is where it, while facing the marine, grabs and raises the human into the air, then quickly impales him through the abdomen with its tail, and releases the marine. Unsure of any of the others since ive only just got the game.