From Quake Wiki
"Thou canst not kill which dost not live. But you can blast it into chunky kibbles."
Zombies are truly unique. They rip pieces of flesh off themselves and throw them at you for their attack. To kill them, you must gib them. If you do not, they will simply fall over and get up later. To gib them you will need to bring their health to -40, meaning you need to 120 damage in a single instance, this is most easily done with explosives as demonstrated in demo1.dem but can also be achieved via Quad Damage attacks with other weapons, or a Thunderbolt discharge of at least 3 cells.
"Player joins the Zombies"
|Monsters from Quake|
|Chthon | Death Knight | Enforcer | Fiend | Grunt | Knight | Ogre | Rotfish | Rottweiler | Scrag | Shambler | Shub-Niggurath | Spawn | Vore | Zombie|