User blog comment:Xilinoc/Ch. 557 - He Already Set His Life Aside/I Already Threw Away My Life/@comment-24113610-20131113162859/@comment-11051098-20131113195411

I was kinda surprised how Bambietta didn't understand such a simple explanation. Sajin sacrificed his heart to obtain a human form and gain more power, and as of such, his Bankai shed its armor and became "undead", and she can't kill it because it's not alive.

Doesn't seem so hard to understand.