I had that idea a while ago. What's the worst murder weapon? A blunt instrument. "Wouldn't that take, like, months to actually kill them?" "Hehehe... exactly."
In GTA: San Andreas, there's one mission where you have to swim out to a tanker boat at night, in a fog, diving if you're caught by the patrol boats (making them think you're a seal). Then, you sneak onto the tanker with a knife and come up behind the crew and stab them in the neck, one by one. Then you reach the captain, who you can't sneak up on, who throws you a katana and demands a fight to the death. You win, leaving no witnesses, then you dive off the now bloody-covered empty boat into the water... perhaps taking out the people on the patrol boats while you're at it. For this mission, I put my bodybuilder CJ in long black pants, a black tank top, and a hockey mask. I called him, "The Black Ghost". That might have been the most fun I ever had with that game.
I want to make a game like this, you know, where you're a serial killer on the loose, avoiding being caught and filling the public imagination with fear over your methods and grim title. If you do well enough, you can murder from beyond the grave a la Freddy Krueger. It'll never be made, but it's a fun thought.