Some things to add from the notes:
Telling someone to kill themselves, even as a joke, is against tumblr's community guidelines and you can and likely will be reported if you do it publicly.
As for doing it privately, while your friends probably won't report you, tumblr knows which asks and messages are connected to which accounts, EVEN ANONYMOUS ONES, so if you send stuff that that to someone else it can be traced back to you no matter what.
Which leads us to two, this kind of behavior can land you with actual legal charges if the person you sent that message to actually does take their own life. Even if they're a friend and you were kidding, their family might not see it the same way. And even if the person you're saying it too is your friend and knows you're joking, well if you have a really bad falling out they have dirt against you. Or if one of your accounts gets hacked, then whoever did that has even more dirt on you. Trust me, that shit looks Really Fucking Bad out of context...and in context, tbh. It just looks bad.
Like seriously, I am usually pretty chill about dark humor, but this is literally the One Thing it is hella irresponsible to just say to people for dozens of reasons and tbh you should work on removing it from your vocabulary asap. I know people don't really do the whole internet safety stuff anymore since the government sold our souls to Facebook and Google but there is no real anonymity online anymore and "the things you say online can have actual repercussions for your real life and the lives of the people around you" is a very basic rule that is more true now than it ever has been before and I think we all need to remind ourselves of from time to time.
TL;DR: Don't tell people to kill themselves, even as a joke. It makes you look like an asshole, can get your account terminated, can get you in trouble with the law, and is all around just a shitty thing to do that should absolutely NOT be normalized or treated as a joke.