Actually , as executions go, it would be unusual, surely, but not "cruel" It would surely be near instantaneous. No pain, just instant disintegration. The executee would feel no more pain than the successful detonator of a powerful bomb vest would feel. No lingering.
Agreed.
However, the “lead up” to the event might involve more than a little bit of “anxiety”. Could this be considered cruel?
Maybe a little...
I was just going to comment on the same line but you said it better
The cruelty is to the audience. Surely Kim did not waste an opportunity to demonstrate to a few hundred of his closest “friends” that he means business.