"kill me" isn't funny, isn't cute, isn't 'a mood', it's fucking serious shit. don't make light of suicidal ideation.
as someone who deals w depression and suicidal ideation on the regular and has friends who deal with it on the regular: this is just dark humor for some people.
there are plenty of times when im feeling fine, a friend suggests something dumb, and i'll jokingly say i'd rather die. a creep hit on me and i literally said he'd have a higher chance if he was an actual serial killer.
it's cool if ur uncomfortable with it and u should tell people that (and they should respect u for not liking this) but im sure plenty of people saying kill me are probably dealing with the same issues and just making light of their own shitty situation.
(i could get into how this can however be an unhealthy coping mechanism but ill spare everyone)