What makes a prank funny?

I just watched two drive through window pranks perpetrated by the same guy. The first left me gasping for breath from laughter and the second made me sick to my stomach. Then it occurred to me that because so many of the variables are kept the same in both videos (drive through, unsuspecting late shift workers, prankster, etc) that we have a semi-scientific experiment here. What are the key variables that make one prank hilarious and the other cruel and inhuman?

Prank number 1: Expertly crafted car seat costume that makes car appear driverless.

Invisible Driver Prank: Hilarious

Prank number 2: Static shock generator zaps the worker followed by the driver either passing out or going into convulsions, usually driving the car in the process.

Static Shock Fainting Prank: Cruel and Inhuman

What's the difference? I see two big ones.

The first is the role of the prankster. In the invisible driver prank, the prankster merely sets up the situation and watches the action as a passive observer. In static shock, the prankster is an actor who creates the illusion of a very serious problem. In this case, the prankster actively misleads the victim (and they are truly victims in this case) into thinking they have caused serious harm and possibly killed a man.

The second is the response the prankster is hoping to illicit. The response in the invisible man prank is the hallmark of a good prank in my mind: confusion. The staffers are shocked, confused, question their state of mind in some cases, but always end up laughing at themselves and bring other people in to enjoy the moment.

But in the static shock prank, the emotions inflicted are fear, terror, distress, shock, and anguish. There's no moment in which he reveals the prank, he maintains the illusion that he's in great distress caused by the victim throughout the video. He leaves the scene with the victim still believing he/she has seriously hurt someone. I find no humor in this whatsoever.

This isn't to say a prank shouldn't scare someone, but if the prank creates a victim rather than a participant it should never have been perpetrated.  The best pranks leaves everyone involved laughing.