This is the worst thing you can possibly say to someone here because it is the _only_ thing that can lead to actual harm. It doesn't matter if you're the enemy Kayle telling me to do it because I'm beating you 5-0 with Garen, or you're my ally telling me to do it because I died to a counter jungle as Warwick. If I'm in the state of mind where I've actually been contemplating suicide-- and since you all don't know my name it's okay to tell you that I have been for quite some time-- the words have weight.
Simply seeing those two words aimed at me crushes me a little more, cuts a little deeper despite the fact you don't know my name, I don't know yours, and I know you don't know how much I really want to. I've even taken to responding jokingly with the all-too**-true** statement,
"I was going to, but someone called the police."
I don't think it needs to stop because this community is full of assholes-- the kind of game this is dictates that the community will always be full of assholes. I think it needs to stop because it has stronger implications than anything else you could say. Call me a bad player, fine. Call me a slur that doesn't actually apply to me because you don't know me, fine. But when you tell me to kill myself, you don't need to know me to hurt me.
Edit: I've noticed there are a few people coming out of the woodworks to say, "stop being a bitch." Since that solution relies on the premise that this is a matter of strength, it's incorrect in its base assumption and needs no further rebuttal. All I'll say is that I actually agree when it comes to offensive things. Merely _offending_ someone should never warrant a punishment, since offense it entirely subjective. But this isn't offense, it's objectively an attack on the person. The one condition is that you may be in ARAM and need to buy, but even then the time difference between typing "execute" and "kys" isn't an excuse.