This is really simple, even when abge was in charge of the forum he did not allow threads specifically calling out other members. A thread that begins by saying that we should consider imprisoning and killing people like a specific member is so clearly out of line that I'm fairly concerned that it seems multiple people don't understand why it's inappropriate.
"my intuitive guess is that zultar got a few PMs, read the opening post(s) and closed it without really taking the thread as a whole (it was a meaningful discussion, I learned something, and some of the replies were very thoughtful.)"
This is wrong btw, he, myself, and 2 other mods where in a gchat conversation reviewing Zultar's post per his request. I can affirm that he'd read the entire thing and was well aware of what was said inside it. This is similar to sexual based conversations. While explicitly detailed posts are allowed on the forum it isn't allowed in the OP, from when Kestas and abge made a decision on that issue. If you want to have a serious conversation about a topic then you need to do so in a way that doesn't leave members thinking that you might seriously kill another member if the opportunity arose. That never has, and never will be acceptable, as our members are here to play diplomacy, not to worry that someone they meet here would seriously consider doing them harm if the opportunity arose.