Look, it's very simple. You don't get to choose how people react to your words. You're expected to exhibit a common courtesy to have a reasonable expectation not to piss people off.
It's not purely about "taking offense". If I'm playing a game and doing my best, but you take it upon yourself to call me a "boosted idiot" and do your best to make me feel bad about my performance while spamming "?" pings on me, you've robbed me of some amount of enjoyment of that game. Sure, I can mute you once I determine that you aren't stopping, but then maybe the same thing happens the next game, or the one after that.
Am I going to devolve into a puddle of tears? No, of course not. But maybe I don't feel like logging in for some games the following day, or the following week. Folks going out of their way to make me feel worse about my game performance chips away at my desire to actually play the game. Maybe I take a long break. Maybe I quit. Riot sure as hell is getting less RP purchases out of me.
Ultimately, this is a game and we're supposed to enjoy playing it. Being an asshole in chat acts directly AGAINST that fundamental, basic idea that games should be fun.