Then the next level will be them reading the vibrations from the screen caused by your voice.

If society says we have to execute (not my opinion due to errors in convictions) then how about putting the condemned into an airtight cell where we can control the atmosphere. When he is asleep then roll a Bingo cage and if the DEATH ball comes out, then pipe in the nitrogen. He knows he is condemned, but not the exact day, and his luck will run out soon. One day he is just fine and the next - well, he just doesn't wake up.

'Liquid metal' doesn't necessarily mean at room temperature (unless you've been very naughty). The sodium filled exhaust valves in my old 914 only have liquid centers at operating temperatures.