Fatal Blow

One black mana for an unconditional kill would be a mispriced card, so the conditional carries the entire bill: the creature has to already be wounded this turn before this can land the finishing strike. Most black removal pays for its kill in life loss, restricted targeting, or a stiffer mana cost; this one pays in setup, usually a combat block, a chump trade, or a separate damage source first. The reward for clearing that hurdle is steep efficiency and a hard "can't be regenerated" line that walks through the regeneration shields that defined creature defense in its era. The instant-speed window is where the conditional earns its keep: you can hold it through declare blockers, let the damage resolve, and pick off whatever crawled out of the exchange below its toughness. It cannot start a fight, only end one, since something else has to draw the blood. Black's flexible kill spells answer threats on their own terms; this one only rewards a board you have already shaped, which is precision in exchange for proactivity. Note what the play pattern actually buys you. Finishing a creature your survivor already damaged is a clean one-for-one. Finishing one that traded with a creature that died is two of your cards for one of theirs. The efficiency lives in the rate, not in card advantage.