This works too... I still prefer your initial attempt though. I liked the imagery of a curse that lay over the target for the duration of the spell.Ok so trying a different angle. Instead of trying to create a bunch of language to protect the caster from wasting the spell....what if we just prevented the caster from wasting the spell?
Power Word Kill
Range: 60 feet
Casting Time: Bonus Action
Target: One creature you can see within range
You utter a word of power that can compel one creature you can see within range to die instantly. If the creature you choose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect, but the spell slot is not expended.
So in this version, you waste the bonus action from the casted spell (and couldn't cast another non-cantrip this round as normal) but otherwise can use the spell worry free about wasting the slot. The spell with either work, or you "get your money back"