See, the problem with this idea of death by squishing is the mess. Executions shouldn't have to have a huge amount of clean up when they're done properly. If you go squishing a guy's guts out you'll get all this gooey stuff everywhere, and have to wash, disinfect and then make the weight ascend again. All this work, jeez an axe would be easier to make from steel.
Another part is that the dead man walking can probably hear the thing falling onto him. Giant discs made of metal have a noticeable 'woomf' type sound when they drop. That person would likely scream as they detected the drop. No one wants to hear the man scream.
Violence is all good and well when you're an angry mob, but it doesn't do when you're trying to maintain a civilization. To keep the proles afraid you need the Death Penalty, but to keep their minds off death you want it to be forgettable and a non-event. If they get all excited for sentencing someone to death they'll make up shit in order to watch someone get killed.
Thats why I will rather prefer to be subjected to the piston helmet model, with the additional twist that the sentence is declared executed in the moment said helmet is worn and the piston is activated by a timer randomly set to activate the piston in the next minute, the scare/revenge effect is still obtained in the public wile not vexing the executed.
I agree wit William Blake about the death penalty is a huge admission of failure, a flebile deterrent and bad for the image of any state...
But if You want to go ASB I prefer confession The Sword of Truth stile to the fate that lurks in room 101