It seems that I joined the same game as you. I got regularly killed by some Predators. Some people don't get coop part of the horde style game.

Although you will not be able to do this when joining games, if you are hosting just simply set the game with friendly fire off.

That being said, if everyone agrees to it, having Marine vs Predators vs Aliens AI is a tonne of fun.

What Grzegorz said. If you want to play it "the way it's meant to be played" (even if AMD user like me :P), host a COOP yourself and turn friendly fire off. I've only played DM so far, but I've seen quite some comments that every game seems to end in a deathmatch anyway, because people start to shoot everything that moves. So hosting with friendly fire off seems to be a good idea.

If I'm playing and someone kills me repeatedly in co-op, I do my best to target them and send them to the netherworld for a few seconds. But usually, there's unintended casualties.

My first GOG server coop game I chose to be a Predator and had 3 marines ganging up on me. I slaughtered them all repeatedly for the next few minutes and quit, it just wan't fun. However it seems to have gotten better, most of the friendly fire incidents I've had were short 2-3 round bursts because something was running down a dark hallway. Once that something is identified as a fellow Marine everyone stops shooting. The only exception was in a game where I got my hands on a minigun and a Predalien was running at me. Naturally I unleashed hot fury on the sucker, when some poor Marine jumped down from an upper platform right in my line of fire. Both the Marine and the Predalien were reduced to a mess of twitching pixels. So I would like to take this moment to formally apologize to anyone I have, or will, kill by accident.