the.com/killing

means The act of causing death — or, loosely, a staggering windfall of profit or success, as in 'making a killing.'

from From Old English 'cwellan,' meaning to kill or murder, part of a Germanic word family that also gave us 'quell.' The slangy sense of a 'killing' as a big financial win is much newer, emerging in informal usage as a vivid way of saying you'd scored a win so decisive it was almost violent — you'd slain the market, the deal, the game.