a couple of em got nowhere else to go
4 of em just want to piss around with their immortality shit
actually looking back on it...kisame left his village because he found out a secret of the mizukage, but the secret was that tobi was controlling her/him. but....why'd he leave just to join the guy's groups anyway.
the guy between nagato and yahiko...is he canon I don't remember him at all.
they really all just acted like mercenaries with nagato ostensibly as the ring leader until they could put their actual plan into play.
not to mention they were all just being played by zetsu the entire time anyway to get rabbit bitch resurrected