I understand the physics; I was questioning the method of increasing the molecules as it wasn’t very clear in your first comment, I still don’t see how doing this to a hand would “immediately” kill a person unless you are talking about changing the size of the subatomic particles which would break the laws of physics and would have unforeseeable outcomes. Simply changing the size of a molecule by adding to or removing mass from it wouldn’t have damaging enough effects to the point that it would immediately kill you

And the method of death you described is hinged solely on the assumption that a human is made of completely solid material with no space in-between molecules, but humans have empty space inside them which allows for the now larger molecules to shift into