You can’t go wrong with zombie-killing games.
Either Left 4 Dead 2 or No More Room in Hell (which is free), depending on if you want a more ‘arcade’ or ‘realistic’ feel.
Or you can play Worms.
While not specifically ‘Steam’ - If you’ve got the old Dooms, stuff like Zandronum lets you play through the storyline with up to 64 players. You’d probably have to tweak the difficulty for that.