The last two pages were mostly huge posts of arguments. Each member was pointing out his/her point and all was well, however, more and more started to actually /miss/ the point, and the discussion was turning out to be TOO serious. Serious to a point in which one or two members might be angered or offended and have this turn ugly.
The discussion as you said was going smoothly enough up until now. There was no obvious reason to lock the thread.
However, if you'll notice, at each page, the discussion seems to become even more heated. Members who take this sort of thing really seriously (whether they support the kill the abusers or the find better punishments 'factions') were starting to lose it.