"randomly slice" Scimitar counter bug

It's because the slicing effect is part of the attack animation, it's not technically a death animation. Death counters are coded to have higher priority so that enemy scripts don't break. That's why Odin and X-Zone have the "delay counterattacks" special property. Some spells that remove the enemy sprite lack this spell effect, and the Scimitar isn't a spell at all so it can't be given the effect (you could probably rewrite the scimitar code to add it in, though).