Go for the Throatremoval$0.49Efficient instant-speed creature removal in black at the same general mana range. It is narrower than Hero's Downfall because it cannot hit planeswalkers, but it often costs less and is a staple upgrade in creature-heavy metas.
Infernal Graspremoval$7.00A very similar black instant removal spell that cleanly kills almost any creature. It trades the planeswalker mode for a lower cost and unconditional creature removal, making it a strong budget and tempo option.
Power Word Killremoval$0.12Cheap instant removal that handles most non-angel, non-demon, non-dragon, non-eldrazi creatures. It is less universal than Hero's Downfall but serves the same defensive removal role at a lower cost.
Cast Downremoval$0.67Efficient black instant removal that destroys most nonlegendary creatures. It fills the same removal slot as Hero's Downfall in many decks, but it cannot answer planeswalkers or legendary creatures.
Vraska's Contemptremoval$0.39A black instant that exiles a creature or planeswalker, giving a stronger answer than destruction against indestructible or graveyard synergies. It is a direct functional cousin, though at a higher cost and with incidental life gain.
Bedevilremoval$0.45Shares the planeswalker-killing role and adds artifact destruction, making it an even broader removal spell in black-red decks. The higher color requirement keeps it from being a direct replacement, but the function is very similar.
Vindicateremoval$0.95A classic unconditional answer that can destroy any permanent, including creatures and planeswalkers. It is slightly more expensive in color commitment and lands in a broader role, but it overlaps heavily with Hero's Downfall's utility.
Dreadboreremoval$0.42A sorcery-speed removal spell that destroys a creature or planeswalker, nearly identical in target scope. It loses instant speed but is otherwise one of the closest functional analogs.