"Kill", "murder" or "slay"

One can kill any living being, whereas murder is reserved for the killing of human beings. Murder usually implies malicious intention, whereas killing can be accidental or spontaneous. Slay implies killing with violence. It is often used with dragons: to slay a dragon.

Kill - the ending of a life.

Slay - the deliberate ending of a life by another party.

Murder - the unlawful ending of a human life by another party.

Commit suicide - the deliberate killing of oneself.

Execute - the legal (or sanctioned by some authority) killing of another person.

There are many other words to describe killing.