"For one, if your mental issues are causing you to be abusive to others that's not riots fault to fix" - totally true, but i would not call telling kys once or even spamming kys abusive, the limit is waaay higher for me to call something abusive...
Telling somebody to kill themselves is the worst thing you could ever say to another human being.
If somebody would tell me "kys" in chat, I honestly wouldn't get hurt. However, that will bother me on a moral level.
Not every player is indifferent to trashtalk. Some are sensitive and will take insults to heart.
Furthermore, when is telling someone to end their lives trashtalking? Trashtalking is getting your opponents to tilt.
Go ahead, call them bad. That's on you.
Insulting someone for who they are or telling them to kill themselves is a whole different matter.
I know most, if not all players who say "kys" don't really mean it. So what? Does it erase the fact that you've just said to someone else to harm themselves?
Just think for a second. What if you said that to someone who just wanted to have some fun playing League and already is mentally exhausted?
You just don't say that. In fact, you should never say those words to another human being, even if they are litteral human garbage.
Why? Because you need to respect others. Be decent. No need for you to be overly positive. Just behave like you would in front of strangers.
Do you tell strangers to kill themselves? Do you tell strangers they're worthless human beings? Do you tell strangers they're trash?
No, you don't.
Just because it's a videogame and it's the internet doesn't make it right to normalize trashtalking and stuff like "kys".
If it's innocent banter, sure. If you and the ones trashtalking are on the same page, do whatever you want, as long as you're not harming anyone.