Hired gun

A mercenary is a person who takes part in an armed conflict who is not a national of a Party to the conflict and "is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party".
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Noun
1. a professional killer who uses a gun
(synonym) gunman, gunslinger, gun, gun for hire, triggerman, hit man, hitman, torpedo, shooter
(hypernym) murderer, liquidator, manslayer

slang. มือปืนรับจ้าง (ฆ่าคน), ผู้ลอบฆ่า