To kill all creatures on one level

Push the ~ then type in ode to jack.

Multiple cheat codes

To bring up the console puhs the ~ (tilda) key and type the following codes,followed by the enter key.

fly-toggles - fly mode on and off. to fly press the jump button.
ode to jack - kill all creatures on stage.
impulse 11 - godemode
noclip - toggles clip mode on and off
impulse 10 - large magic attack