Answer: It’s a simple shorthand for suicide, usually indicating a humorous sentiment.
The meaning of the abbreviation is “kill myself,” usually in a joking manner that implies “don’t worry about me – I don’t really want to kill myself” or something similar. Sometimes it just means kill me quickly because I’m so angry, confused, frustrated, devastated…you know how it is. In any case, the abbreviation isn’t used frequently outside of certain popular forums and IRC channels on the internet. Otherwise you might be better off asking elsewhere what KMS means in terms people might have more knowledge about other specific contexts [honestly we’re finding this answer hard to write].