killer

baneful, deleterious, destructive, harmful, injurious, noxious, pernicious, truculent

infectious, infective, pestilential, poisonous, sublethal, toxic, virulent

dangerous, grave, grievous, hazardous, jeopardizing, menacing, parlous, perilous, risky, serious, threatening, ugly, unhealthy, unsound

bloody, internecine, sanguinary, sanguine

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also problematical), recondite, serious, spiny, stubborn, thorny, ticklish, tricky

bruising, burdensome, exhausting, labored, onerous, oppressive, stressful, taxing, tight, trying

annoying, bothersome, distressing, irksome, troublesome, vexatious

grievous, grim, strict, stringent

brutal, cruel, inhuman, painful

drudgery, grind, heavy lifting

effort, strain, sweat

burden, load, weight

bother, nuisance, trouble

bravo, hit man, triggerman

butcher, executioner, massacrer, slaughterer, slayer

murderess