I'm still partial to the idea of hero points. They have the bonus that they can be used to avoid other instakill situations, all in one elegant mechanic. They also allow you to make progress even if your instakill spell doesn't slay the target, because you're still depleting their resources.
Spend 1 hero point = unaffected by power word kill
Spend 1 hero point = unaffected by medusa gaze (for 1 round)
Spend 1 hero point = avoid bull rush that sends you over the cliff
Spend 1 hero point = land the killing blow on the BBEG before it takes you all out
Spend 1 hero point = avoid the killing blow from the annoying hero, thus allowing you to take them all out
Spend 1 hero point = unaffected by power word kill
Spend 1 hero point = unaffected by medusa gaze (for 1 round)
Spend 1 hero point = avoid bull rush that sends you over the cliff
Spend 1 hero point = land the killing blow on the BBEG before it takes you all out
Spend 1 hero point = avoid the killing blow from the annoying hero, thus allowing you to take them all out