You Are Already Dead
Kill spells this cheap always carry a leash, and here it is a timing puzzle rather than a mana tax: the target must have taken damage this turn and survived it. That last word does the constraining. A creature that soaked up lethal in combat is already gone to the rules engine before you ever get priority, so this cannot mop up trades that resolved themselves. It wants a survivor: the blocker that ate a smaller attacker and walked away scarred, the beater a ping softened but did not finish, the fatty a burn spell brought within finishing range. Cast into that window, one black mana erases a creature and hands you a fresh card in the exchange. The replacement is what lifts it out of fringe combat-trick territory, though it is not a blank check. The draw only fires when there is a legal target, so a hand with no wounded creature to point at leaves the spell inert, waiting on you to arrange the damage first. That is the honest cost: you supply the wound, the card supplies the finish and the refill. Held patiently, it sits in reserve until a ping, a block, or a burn spell opens the door, then closes an account at instant speed for no net loss of cards. The only question it ever poses is whether you have set up the damage, never whether casting it will cost you tempo.
