Basically, this is how the final encounter went...
The party got to the encounter with the Ice Witch. One of the barbarians in the party felt that some of the people from the towns should be made to suffer and didn't particularly feel the Witch's plan should be stopped. He cut a deal to join her side.
I don't know exact numbers of his character, but I can say he was a mix of Barbarian and Fighter, and focused on two-weapon fighting. His style was basically to throw out as many attacks as he could and then add the rage bonus to all of them.
On the 'good' side; trying to stop the Witch, was my character (halfing rogue/monk mentioned in other threads,) a barbarian, a wizard, a fighter, and a cleric. The wizard -not surprisingly- went down very quickly. What was more surprising was that the regular barbarian also went down rather easily when engaged in a toe-to-toe fight with the traitor. While the regular barbarian was dealing far more damage when he hit, he was getting less attacks. The traitor, between having advantage via rage and a multitude of attacks from TWF had multiple rolls with which to fish for criticals; he also easily made up for his single attacks doing less damage by being able to stack his rage damage bonus on all of the attacks. After dropping both the wizard and the barbarian, the traitor then moved onto my character. The only thing which saved me was that I had a much higher AC (17) than some of the other party members in spite of the fact that I was not wearing armor due to having a high dex and a respectable wis. Really though, even with that, it was just poor rolling on the part of the traitor. I could, to some extend, match the amount of attacks he was putting out, but his damage was higher, and he was still raging and such got advantage on his attacks. In the end, I won out, but just barely, and I contribute it more to his poor rolling toward the end of the session more than anything else.
Had his rolling been marginally better, he would have very likely dropped my character. Then, at that was left was a cleric who was ill suited for melee, and the fighter who had already been badly wounded earlier in the fight. One PC would have very likely killed the rest of the party. While I understand that PvP is not a design consideration for D&D, something about that scenario still seems wrong to me. A lot of things about that scenario still seem wrong to me, and I don't believe it all boils down to "well, that guy wasn't playing D&D right."