Kakuzu (joined because of loyalty to the cause and leader)
Orochimaru (joined for the chance at getting Itachi's eyes and his grudge against Sasori)
Konan (of course is on Pain's side)
Hidan (joined because of Kakuzu)
Kisame (joined because of loyalty and Madara's order to watch Itachi)
Sasori (joined out of hatred for Orochimaru, and because he had worked with Itachi before)
Zetsu (joined because he is secretly Kisame's ally)
Deidara (joined because Itachi threatened him again with genjutsu)
I tried making the sides a fair a possible.
Pain has his paths of course, Itachi is at full health, Orochimaru is at full health, Kisame has samehada of course, Zetsu has all his bodies, Konan has her reserved jutsu.
Battlefield: Kage Summit
Morals : Typical