People don't like being told they are playing badly. It's as simple as that. There's a disconnect between people thinking "Time to play a fun video game" and "Gotta take this video game seriously because now it's in a competitive scene". People don't reconcile these two things and act accordingly, they get stuck in the former, and it snowballs from there.
Whenever I ask someone to stop playing bad I make extra sure to phrase it in a way that will get my message across.
"Hey Irelia, you've died a lot, I know it sucks but please play safe, we are doing good down here and can come help you later" or "Please play safe, just stop dying and we can carry you bro.". Things like that have more often gotten better responses. Otherwise, just mute them.
The second I see some negative bullshit being typed into chat I IMMEDIATELY tell them they are muted and do it. This accomplishes a dual purpose; the player knows their typing won't be read so it pushed them more towards playing than typing, and keeps MY mental from becoming infected.