Then this would mean he took the damage from his own jutsu (he was spitting a lot of blood from his mouth), which shouldn't be too strange, considering that at that time he had a chunin level chakra pool and freshly reawakened sharingan, perhaps he even misplaced some parts of his body at time, which would explain the need for Zetsu part as well... or just his arms were crushed and cells within them died (blood stopped flowing in them because from the pressure from rocks), thus had to be cut off and replaced.
Originally Posted by Daemon Spade
It's not like Obito had no reason to join Madara. If you look at Anakin's example, then their cases were similar, the only difference is that Anakin joined the dark side, before everything happened, to prevent it (yeah, nice going there), while Obito joined after everything went downhill.
If strong enough, love is all the reason humans ever needed, no matter if it is heroic, or horrific.