Onomatopoeia

Onomatopoeia is a serial killer. He specifically targets non-powered, vigilante superheroes, though he's also targetted supervillains as well as bait or to escape. He imitates noises around him, such as dripping taps, gunshots, etc. and carries two semiautomatic handguns and an army knife. Not much is known about his true identity, but he is a skilled unarmed combatant and possibly metahuman (or at least peak human agility), based on his reflexes.