What to do when one my co-authors is openly insulting me but the others remain silent?
Things have gotten personal, it is best if you remove yourself from the fight. Anything you say, no matter how sensible or innocent, is likely to be poorly received.

Instead, talk with your most trusted collaborator and ask them to defuse the situation (and follow up, to make sure they do!), preferably someone who has received the insulting emails first hand. Once your difficult collaborator has calmed down, you can all meet and discuss the paper. (NB: it is possible that some have already started to talk to him about this in private, or they are just waiting to see what on Earth is happening with you two, so silence doesn't necessarily mean inaction).

I'd say it is rather unusual for someone to snap at you, so there may be something else going on here, and perhaps you should look into it. On the other hand, some people are plain rude. In any case, personal insults are not acceptable, and this behaviour should not be tolerated.