You can’t *legally* kill them but at their age I am sure there are all kinds of plausible mishaps that could kind of just happen to them where death would be the outcome. Nudge nudge wink wink.
Same with getting them certified really. Leave the gas on for them, hide their keys, move things around the house, plant a walkie talkie in a few rooms so they start ‘hearing’ voices and bobs your uncle. Soon tip them over the edge.
INAL and also some or all of what I am suggesting may be illegal and the solutions offered are merely hypothetical and I am in no at all suggesting you should actually carry anything of the sort out.
I would strongly advise you take full and proper legal advice before embarking on any mission to have your parents certified or otherwise ‘dealt’ with.
Cheers
Danny B