Back in 2012 my grandmother went to see the doctor for a checkup and he decided she needed more tests done. After those were done we found out she had stage 3 or stage 4 cancer, mostly in lungs. (She never smoked btw). She deteriorated during that summer and the entire time my grandfather(who also had cancer but it was more manageable due to being less severe and being found earlier) was a mess the entire time. By September of that year my grandmother was on her last legs and the cancer and the treatments pretty much destroyed her immune system. She passed away early that month from pneumonia which she had because of the cancer.
My grandfather never got over it and would often randomly break down. He passed away in early 2014 from what seems to be Natural Causes. But it probably really was from a "broken heart". Even early on while managing his cancer he still suffered because both my grandparents loved traveling and when home my grandfather loved working around their house and land out in the country. When he got his cancer the medicine and treatment made it much harder for him to do any of that and would often get "cabin fever"
My dad also found out he had cancer a year or so ago in his knee, though fortunately in that case it was found early and he's been cancer free for a while. Though the treatments were hard on him too and he's a bit less active now.
So yes wishing cancer on someone is an incredibly horrible thing to do and if I see anyone say something like that in game or anywhere else on the internet for that matter I immediately wish I could punch them through their monitor. Like my fist would literally fly out of their monitor and punch them right in the face. For the time being though, I guess I'll just have to settle with Riot making that one of their automatic 14 day "last warning" ban offenses.