Faithful with Mission and Organization
While on a mission, Kisame can build a relationship with one of the women of the division. Unfortunately, their team was ambushed by the Konoha Ninja led by Ibiki, the Chuunin exam supervisor in the Naruto era.
Not wanting any information from this division to leak into another village, he even chose to kill his friends, including previous women. After all he did, killing his own team, Fuguki sold the information to another village.
Of course Kisame who is very loyal to the mission does not accept with this, and without exception, he kills Fuguki and takes Samehada. Yagura (who controlled Obito) saw Kisame's allegiance to Kirigakure.
He also met with "Madara" and told the story of Jutsu Mugen Tsukuyomi. Therefore Kisame joins Akatsuki, and becomes a runaway Ninja.
Moreover fact coming from Kisame Hoshigaki? Check the next page, yes.