As somebody who set-up and now moderates a very successful Facebook based community forum (group), all I will say is this is a very tricky subject. Sometimes the community will play nice for months and then, for no obvious reason, a series of posts are made that flare up. The four moderators usually have a chat via private message about what to do with the thread. Sometimes we let it run because some subjects are worthy of discussion and a heated discussion often indicates very clearly to the local town council where the hot spots are - usually parking or dog poo 
Sometimes we let people have their say and lock the post. Sometimes the post is deleted esp. if it starts becoming very personal.
We ask that everyone is civil but that doesn’t mean don’t get emotional or heated.
For every bad threads we have many more good threads where the community rallies together to support somebody or a group.
UPDATE: Interestingly I was going to post a link to a thread that blew up today that was about religion but it’s gone so the original poster must have deleted it. Self moderation is always easier!