Cheats:

 Type them in whenever you need them:

 NICK             - Full health
 RUCE             - God mode
 DUSTIN           - Go to level 1
 BOSTON STRANGLER - Go to level 2
 HELTER SKELTER   - Go to level 3
 CHARLES MANSON   - Go to final level
 MURDERER         - Extra guns
 SON OF SAM       - Extra items