Zombieman

The Trooper (also known as the Zombieman) is a marine who has been turned into a zombie during the invasion of the UAC bases. He appears as a green-haired man in bloodied utility uniform, carrying an assault rifle.

The Doom instruction manual says:
 * Just a few days ago, you were probably swapping war stories with one of these guys. Now it's time to swap some lead upside their head.

In the instruction manual the trooper is called a Former Human.

The trooper is first encountered on E1M1: Hangar (irrespective of skill level).

Combat characteristics
The trooper is armed with a rifle that fires bullets, although they are slightly less powerful than the player's bullets. When the trooper attacks, he aims his rifle at the player for about 3/10 of a second and fires a single bullet. The trooper's weapon has a wide dispersal (around 22 degrees), and may miss at medium to long ranges. After firing, the trooper moves again.

He is generally considered the least dangerous enemy in the game; no other monster (excepting Commander Keen) has a lower movement rate, hit point total, or average damage per shot. When killed, the trooper drops an ammo clip containing 5 bullets (10 on I'm too young to die and Nightmare!).

Tactical analysis
An armed player should never back down from a single trooper, although large groups can slowly drain the player's health during the time it takes to kill them all (as with MAP09: Stronghold). The dispersal of his shots means that he is barely a threat at medium to long ranges. In an emergency, or when conserving bullets during a Tyson run, the fist is effective in the short term if the player's footwork is good (two to three blows will knock a trooper down, whilst a berserk attack will frequently gib the trooper).

The trooper's low rate of fire and damage capability make him a rather weak contributor to monster-monster battles. Attrition among a crowd of troopers is generally slow, and distraction of larger monsters minimal.

In many stock levels, most of the enemies in the first few rooms are former humans. Unless the player's primary goal is speed, the troopers' dropped clips can be a valuable early source of ammunition.

Inspiration and development
An early version of the trooper appears in the wad file of Doom 0.4, although the sprite (POSSA) does not appear in the game. The sprite is almost identical to the Doomguy. It is however slightly lighter, and has crudely-drawn square red eyes (to show that he was possessed). The only frame is a full-frontal view, with no rotations. In Doom 0.5, however, the trooper sprites were virtually identical to the finished versions.

Appearance statistics
The IWADs contain the following numbers of troopers:

Doom RPG
In Doom RPG, the trooper appears as the "Zombie" monster class. There are three different forms, identified by color:
 * Zombie Private (brown)
 * Zombie Lieutenant (blue)
 * Zombie Captain (normal colors)