How come people who spend 10 years grinding gets super toxic?

I seriously don't understand this, Basically the most super toxic players that exist in our community is Plat+ and spent maybe 10000 hours on LoL through harsh grinding only to have one game where they type Death threats, Racists threats, family threats and tons of more extreme things to say. I just met a Diamond Vlad **IN NORMALS** that constantly spammed "%%%" in all chat after dying once by a Gold Riven and shortly after the game as I reported him I got a feedback that he got banned. This guy was level 300. Had about 2 millions Mastery points on Vlad and got banned just like that? Why do people who spent so long on LoL, acting like an idiot 1 game like he doesn't understand the rules of LoL happen? And it's not only for this guy, we all know Platinum is the most toxic place within LoL, it's very hard to climb up to Platinum since you need quite the skill to get up there yet, people constantly get Perma banned up there. Why the hell do they act like the don't know the rules after spending so freaking long on LoL? It doesn't make sense.
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