Killing the currently controlled character doesn't work, because an actual competent multiboxer (which all 5 of the mass-boxers can be called with the years of experience), uses their character like a hivemind. You kill off the current head, and they will almost seamlessly transfer control to a different one, while constantly spamming /follow to regroup any units that got spread apart by a single typhoon or similar.
And I always consider "Random Battlegrounds" less like "Random Map" but more like "Random Players". Yes, there can be small groups of premades in a BG, but other than that, the majority of BGs will have an even distribution of random unrelated teammates, who want to win, yes, but CAN'T work as a group because the random teammates won't listen to strategies. In any normal case, the teams are equally "dumb" with maybe one or two people yelling Key-points in /bg chat, like "go sunken ring". Both opposing teams do not require the coordination of a dozen people to win. But once you add in a Mass-Boxer, the opposing team will have to somehow bring a dozen of people on the same mind-set, just to match a single person, who is effectively acting as 20+ people sharing one mind already.
You are requiring a whole team of individuals to work together to win the match, against someone who is being more effective without working with his team. When is the last, or should I rather say first time you saw a mass-boxer trying to cooperate with his team? And I don't mean "cooperate" as in "Wow you guys are ******s, why aren't you killing the southern towers, ur so bad", or "ok I will def, or in other words, sit at the chokepoint and nuke everything that comes through it, u guys attack". I remember just recently having an allied mass-boxer in Isle of Conquest turtling in our base after we broke the enemies gate and he kept wondering "what the **** guys, just push, how are you losing", when every teamfight was basically 20vs40 (minus the few that roam around the map).
A mass-boxer's team doesn't win or lose due to their teammates working together. They win/lose because: 20 uncoordinated characters (controlled by 20 people) < 20 automated characters (controlled by 1 person) < 20 coordinated characters (controlled by 20 people).
Keep in mind, lower max. player BGs aren't affected by the strengths of mass-boxing because you as an individual can exploit the objective more easily, like Eye of the Storm, but it becomes a lot more relevant in cases like Wintergrasp, where there is virtually no restriction (aside from worthless Tenacity) and the map funnels into a chokepoint.