It looks to me like he tried to start a discussion in good faith.
If that looked like good faith to you, we've got very different understanding of the concept. The first post could be generously referred to as that (excepting, perhaps, that it was copy/pasted from elsewhere, anyway, and had other notable problems), but basically everything after that... wasn't. The discussion could have been handled gracefully, but the OP pretty much immediately (literally immediately, really, straight from the thread bumping second post and continuing with pretty much every post after either attacking another person in the thread, denigrating someone involved in the story's character, or something along those lines) started to try to drag the discussion into a fight instead of a conversation. Which happened, and people stopped taking things particularly seriously (to say the least).
Those who cannot handle good-faith, respectful discussions should not take part in them, and shitposting a thread to death should in no way be a way to get the thread locked or the OP banned.
That's true, at least the latter bit. Unfortunately for rex, it was more than just other people's actions that got them banned.
And @Rol (and points that came up while typing), unfortunately, being provoked ultimately won't prevent you from being banned. If you engage with antagonism with more antagonism, you get considered part of the problem. Pretty much the first bulletpoint in the forum guidelines. Considering rex both had a history of doing so, and explicitly stated in that thread that they were setting out to entice and give out said antagonism, well, the obvious happened. When it comes to stuff like this, the OP nets more of the blame -- they have the tools to stop dogpiles (locks, reports), and the responsibility to set the tone of the thread appropriately (by not escalating or sniping back if people are being offensive and reporting, as the guidelines specifically state to do). OP in that case failed on both points, which combined with other things lead to the ban.
... it's just. Look, folks. If you want to talk about stuff like that thread was, you can. You just have to handle it correctly. If you know you're going into a controversial topic, you can't let the tone of the discussion get dragged down. If you see it getting out of hand, you warn
appropriately, possibly lock the thread of your own volition and report troublemakers if that's not enough, not contribute to the problem. As the OP, you have to manage the tone, start with and maintain decorum, and respond appropriately probably inevitable antagonism. Rex failed on absolutely every point of that, and as the OP and the one that took on the responsibility for the tone of the thread when they made it, got hit the hardest for doing so. Other people probably deserved warnings or mutes at the least, but the targeted ban wasn't particularly out of line.