to kill: to put a person (or creature) to death by violence
This is the plain, all-purpose word for taking a life by force. On its own it says nothing about whether the killing is right or wrong: the same word covers a soldier cutting down an enemy, a court putting a criminal to death, a jealous man murdering his brother, and God striking down the firstborn of Egypt. What it always names is the violent ending of a life and the responsibility of the one who brings it about.
Because the word itself is silent about justice, the moral weight comes entirely from the surrounding story. Cain killing Abel and Israel killing the kings of Midian use the very same verb; only the context tells you that one is murder and the other is war. The senses below are not different words but the different settings in which one act of killing happens, so a verse can sit in more than one of them when the setting is mixed.
The word also turns up in its passive form, where the focus shifts from the one who kills to the one who is killed: the slain, those put to death, the people cut down. Hebrew poetry in particular lingers on the slain as a vivid picture of devastation.