apsuman,
Here's where we get into some basic D&D assumptions: I suggest that the aliens in the comic you mention are more accurately modeled by orcs, goblins, or other humanoids. Since the basic D&D paradigm involves such beasties as mindless servants of evil with no hope of redemption, it becomes OK for good characters to slaughter them wholesale.
So the aliens in this situation are "killable" because of their basic nature, not their vocation.
Here's where we get into some basic D&D assumptions: I suggest that the aliens in the comic you mention are more accurately modeled by orcs, goblins, or other humanoids. Since the basic D&D paradigm involves such beasties as mindless servants of evil with no hope of redemption, it becomes OK for good characters to slaughter them wholesale.
So the aliens in this situation are "killable" because of their basic nature, not their vocation.