Shanadeus said:Because he knew he'd get taken over by the spell and become a "mindless" zombie - he was after all disobeying orders so he might not have been able to disobey them long enough to be sealed.
Instead he just disobeyed the orders for a split second so that he could paralyse himself long enough to be sealed even if his mind was later purged.
Why would any of that be neccesary when there was a window of opportunity after Mifune slashed Hanzou instead of having a nice conversation, plus Kabuto tried to switch Hanzou into full puppet mode after he commited seppuku.