lethal weapon; deadly device
Refers to any weapon or device designed or capable of killing people. Can be used literally for military hardware or figuratively for something extremely dangerous.
It was reported that the country is developing a new lethal weapon.
This knife is no longer a tool; it's a deadly weapon.
凶器 is a more common and general term for a weapon used in a crime, while 殺人兵器 specifically emphasizes lethality and is often used for military or large-scale weapons.
Compound of 殺人 (satsujin, 'murder, homicide') and 兵器 (heiki, 'weapon, arms'). The term is straightforward and modern.