Kakuzu's strange eyes, they're exactly the eyes a dead has when is called to life using edo tensei. Look to this image, by the time orochimaru summoned the 1st and 2nd Hokages. After the sealing done by the 3rd, their eyes returned to normal and they died (again).
And he said a very particular thing:
This means Kakuzu has already been in hell, and was probably brought back by a edo tensei jutsu.Wouldnt it be ironic, Hidan being immortal, and Kakuzu being dead?