Here is a list of all the people and aliens you need to kill

The Enemies 
Name Description Picture
Headcrabs These things jump at you, attach to your face and turn you into a zombie, if your don't kill them quick.
Zombies This is what happens when one of the headcrabs turns you into a zombie. If you get too close, they'll lash at you with their claws. so stand back and shoot them in the head.

Bullsquids These just spit at you - well, at a distance, but if you get too close, they'll make you go flying!
Alien Slaves These aliens can pack a powerful, electrical punch if your not careful!
Houndeyes These use waves to hurt you, but aren't that hard to kill.
Barnacles These things don't really do much, unless you walk into it's tongue, in which case it will pull you up to it's mouth and eat you. Just shoot them a few times and you'll be fine.
Alien Grunts These are very powerful, and are pretty much the only alien that uses a weapon. Shoot them enough times and they'll go bye bye.
Ichthyosaur These are quite difficult to kill, but compared to the aliens below, they're easy! They move very fast through the water, too.
Tentacles These are very strong and cannot be killed by regular weapons. They do not have eyes, so to get around them you just have to crawl very silently.
Gargs These aliens are another in the can't-kill-with-weapons variety. Just stay away from them as much as you can.
Alien Controllers Control aliens I assume, though they seem to just float around firing weird blobs at you more than anything.
The Military The good news is the military are here! The bad news is they will kill anything they find! And that includes you.
Trained Assassins They're fast, they're black (clothed), and they've got good guns! These are the hardest human opponents you'll come across. They're too fast to take a picture of!