I'm surprised there's only been one Isaac Clarke suggestion. Clarke not only kills aliens, he kills zombie aliens. By tearing them limb from limb. Then, just to add to his badassery, he uses their own arms to impale their buddies. On top of that, he survived not one but two infections. He effectively killed an entire ship and planet's worth of mutant alien zombies, but then went to the most populous space station this side of Vega and wiped out its population of alien zombies. If that weren't enough, he does it using a mining tool. And, if that weren't enough, he does it without an ounce of military training. He can and has taken down explosive babies, animated needle-shooting fetuses, swarming mutated school children, blade-armed vent-crawling reanimated corpses, enemies that explode on you, enemies that barf up corrosive acid, Jurassic Park Velociraptor-equivalent aliens, giant monsters, immortal enemies, animated heads that would love to rip off your skull and replace it with themselves, and so on and so forth. That's a badass.