I really don't understand what happened between his fight with B, and this arc.

I mean, I could understand him taking a nosedive after finding out what happened to Itachi, but he was still kinda the same after that when fighting B, but then in this arc, he's become an irredemable bastard.

Between this sudden character shift, and him suddenly learning a new Summon, I almost get the feeling like some kind of story arc was skipped over in the middle here somewhere.
Well the difference is that capturing Bee was a mission for Akatsuki, while killing Danzo and anybody who stands in between was his own personal revenge against the Kohana authorities responsible for the Uchiha incident.