It is not possible to kill Playboy X in Grand Theft Auto IV. He is a mission-critical NPC protected by script immunity.
Choose Kill Playboy X in "The Holland Play" mission. Dwayne lives; you get his safehouse (Northwood) and can call him for backup (2 carloads of gunmen). Playboy dies; $25k lost; no further contact.
That's the *only* kill. The game forces the choice—it doesn't let you cheat it.