Heres the truth. People who play games competitively will flame, no matter what. The pros flame their teammates when they fuck up. Now some people take it over the edge with like death threats and slurs, that can never really be justified because it isn't socially acceptable anywhere. But normal flaming is just a way of venting pent up frustration, and it's psychologically speaking, healthy for the most part. I flame my friends I play with IRL. If we are playing in a competitive way...that is key. A casuals game, it doesn't really matter so theres no point to investing any passion. But if you go into the game with a hard set goal to win, and you have a backbone..u will flame if someone fucks up or is argumentative. It happens in corporate america, it happens in households. Those people still end up working with those people the next day. People do not behave like assholes in true high elo, because they either don't care because they know they can get their LP back next game...or they are in fear of being banned.
If you see a toxic smurf they are probably just venting pent up rage they can't vent on their main. Seems like thats a sentiment they communicated here. If this upsets you enough that you wanted to get back at them, just submit a support ticket with that text you provided. Should be a perm since the perp admitted to sharing an account. Personally, I don't find any of that to be worth my time. Sometimes I have some fun flaming back telling them how much of a scumfuck bastard they are, but I don't really care to take more than the few seconds to tell them that.