If person (A) feeds and trolls person (B) in a game and makes them lose then yes, I think person (B) has every right to go off on person (A). Saying or doing nothing makes person (A) think it's completely ok to troll when ever he wants. The problem is , everyone on the internet is a wanna be Badass and takes nothing serious anyone says, accept the sensitive cry babies who pretend to be offended so they can troll even more and report.
At the very least person (B) gets to let off some steam after the idiot person (A) just made person (B) waste 40+ minutes in a game just to lose because he thought it was funny. So yes, it does help.. Rather it does anything or not to person (A) it helps person (B) relieve stress..
In real life if someone did that you would just punch them in the face, kick them from the team or fire them from a job for doing stupid shit like that, but because you can only type on the internet it is the 2nd best thing to relieve stress.
Now saying to someone you hope them or a family member gets cancer is the right thing to say? Probably not.. But if it helps them relieve stress screw it , they earned it if you trolled them or went out of your way to screw the game up. Taking it literally as if them telling you that will somehow magically make you or your family get cancer falls on you.. Don't take shit people type seriously on the internet. If you can't help it then again that falls on you and you probably shouldn't be playing games if you are sensitive to stuff like that.
Riot can make all the rules in the world but they aren't going to change human nature to relieve stress if someone is mad. Most they can do is suspend or ban them in which case that person goes to a different game and Riot loses out on potential money.
Moral of the story? Don't troll people.. if you don't don't cry about it on forums if they say something you don't like. Be happy that's all you got and not an ass kicking in real life. If you don't troll them and they say that shit then just mute and report them.