People shouldn’t think twice about flagging a thread if they think it’s going to turn into a shitshow, even in cases where the discussion is relatively civil. The personal sleights repeated in the OP aside, whoever made them, nobody was in the wrong, but it was pretty clear where it was going.
I know from experience that those sort of rows, especially the ones that drag on for hours and end up with essentially two groups of people jabbing eachother can ruin your fucking day, man. Life really is too short.