Go for the Throatremoval$0.49Efficient black instant-speed creature removal that destroys most creatures for a very similar cost. It fills the same general spot as Murder in decks wanting unconditional-feeling removal at three mana.
Hero's Downfallremoval$0.14Instant-speed black removal that can answer creatures like Murder, with the added flexibility of hitting planeswalkers. It costs the same mana and serves the same interactive role.
Infernal Graspremoval$7.00Cheap, clean creature kill spell that is often preferred over Murder for efficiency. The life loss is the main drawback, but it performs the same job at a lower mana cost.
Rite of Consumptionremoval$3.10Black removal-adjacent spell that can function as creature removal by sacrificing your own creature to deal damage, though it is not a direct destroy effect. It fits the same broad interaction slot in some sacrifice decks.
Doom Bladeremoval$0.29Classic black instant removal that destroys most nonblack creatures, offering a very similar role to Murder with a narrower target range. It is cheaper and often the closest budget alternative.
Cast Downremoval$0.67Efficient black instant removal that destroys a nonlegendary creature, covering a huge portion of the same targets as Murder. It is a common budget substitute in many formats.
Heartless Actremoval$0.19Flexible black removal spell that usually kills a creature efficiently at instant speed. It is similar in function to Murder, though counter-type interaction can matter in some games.
Feed the Swarmremoval$0.79Black removal that can answer creatures and, importantly, enchantments, giving it broader utility than Murder. It is sorcery-speed, but it fills the same single-target removal role in black.