Twitter says they do not permit speech encouraging acts of violence yet permits these tweets and this hash tag to exist on their platform – there are zillions of these tweets:
You can search twitter for the hash tag #killyourself and see this for yourself. This looks pretty awful for Twitter.
Update: Just found that TechCrunch reported on this in February of 2018. Twitter still permits the #KillYourself hash tag 18 months later.