Press the kill button.
Default Binding: Mouse 1
Secondary Kill Button: Mouse 2

Fun facts!
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Gibs stands for giblets.
I gib everything I kill.
Gibbing is fun.
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For grenades:
1. Take out your grenade of preference. It can be a rocket, hand genade, or an MP5 grenade.
2. Drop the grenade on the corpse.

For melee weapons:
1. Equip your melee weapon.
2. For the knife, just chop it into pieces.
3. For the pipe wrench, SMASH IT TO PIECES! Oops, got a little excited there.

Note: Does not always work with bullets.

The Knife
Good for gibbing as listed in the previous section.
The Pipe Wrench
Good for smashing people into gibs. (My favorite melee.)
The Deagle/The Desert Eagle
It has a red laser which increases accuracy, but lowers the firing rate.
Barnacle
You all know this guy from Half-Life! Except you use it to glide around maps with the organic green stuff.
Spore Launcher
This thing is an alien weapon that you use to launch the green stuff on the walls. It uses that as the ammo. Disgusting.
Shockroach
Not much I can say about this. Except the fact that it's an electric cockroach.
Displacer
This is the weapon that you use to teleport to Xen. Also to get health.
Sniper Rifle
It's a sniper rifle. Nothing to say here.
Squad Automatic Weapon
It's basically just a big-ass gun.