The best stories told on Arrow lend themselves to far greater levels of moral ambiguity than other shows within the Arrow-verse, and that means making killing not only possible, but inevitable.
Oliver said it best during the series premiere of The Flash: Barry Allen can be something that the Green Arrow never could — not just in terms of physical abilities, but in terms of morality as well.
Arrow has recently struggled to try and match the lighthearted tone of shows like The Flash and Supergirl, but it should really be embracing the inherent darkness of a gritty, street-level crime drama.