I liked the idea of alluding to the "Nail-in-the-head" image and then waiting 3 strips to actually use it as a punch line. I thought it was awkward, though, how the idea returned. It doesn't make sense that the onlooker would ask if his ex-friend had any last wishes before he exploded - I mean, wouldn't a person assume that the explosion had been a sudden, fatal accident?
Maybe, in the third strip, if the robot had asked the protagonist to hammer the nail into his head without using a threat, then it might work that the protagonist could be reminded later of the nail-in-the-head request through the ex-friend's Last Will and Testament (to borrow the idea from the title of the last strip, and the idea of the "person-reading-the-paper" image). I think it would only work if the request weren't described again before the nail entered the head (just the way you'd done it). It's making me think of some alternate ways to end the strip, too, but they're more than a little tangential.