I didn't see Ben as an underdog, more someone who dug themselves into a hole by being so unlikable, like a Russell Hantz. That, combined with his manic edit (where every time he needed an idol or clue, it would somehow fall into his lap) and his over the top theatrics (playing idols early, miming explosions) made him one of the least appealing finalists ever.
I do get the PTSD thing, and that it has made him socially awkward, but I don't see it as justification for unnecessary arrogance.
He betrayed his main ally (Lauren), and alienated himself. Having said that, she still voted for him to win.
Speaking of the jury votes... I feel that was another flaw in this season. Only eight jurors, and the 5 votes for Ben were from Lauren, Cole, Desi, Joe and JP - the latter three being what I would term nobodies in the game, especially Desi who bizarrely had more to say in this episode than she ever did before she was voted out.
The people who were there the longest definitely didn't want Ben to win - Devon voted for Ryan, Mike and Ashley voted for Chrissie, and Ryan and Chrissie would have voted for each other. Maybe that comes to bad jury management, so Ben can't be blamed for that at least.