As for Kisame, he was relatively neutral. Although Kisame was a missing-nin, he was extremely loyal. As long as the faction he belonged to did not betray him, he would not betray it—even if it meant dying.
Nagato's betrayal of the Akatsuki was an indisputable fact. The new leader, Uchiha Madara (Obito), was famous and had never done anything to betray the Akatsuki, so Kisame chose to stay.
"Nagato betrayed us. All those things you did to help him back then would have been for nothing. As far as I know, if you hadn't treated him, he would have died from overusing the Rinnegan."
Obito lightly tapped his fingers against the stone table.
"If you're dissatisfied with me being the leader, I can give the position to you. In terms of seniority, you're no less qualified than anyone here."
Obito wasn't flattering him. Forget everything else—when it came to capturing Tailed Beasts, Su Xiao had first captured them alongside Deidara and Sasori, then joined Itachi and Kisame's team, participating in many Tailed Beast captures.
If that wasn't enough, what about attacking Konoha alongside Nagato? Whether Nagato had betrayed them had nothing to do with Su Xiao. When Su Xiao had participated in the attack on Konoha, he had done so entirely in the name of the Akatsuki. They had attacked an entire ninja village.
As long as Su Xiao spoke up now, he would very likely become the Akatsuki's leader. Deidara would probably support him, while Obito definitely would.
Was becoming the leader really a good thing? Of course not. The Akatsuki currently had only six official members, counting Su Xiao himself. In other words, even if he became the leader, he would at most be managing five people.
No, Obito would never obey his orders. Black and White were