Just a little theory: from what I had read so far, the Cheat Skill "Zombie" is it own entity. It's possible that Yui-chan, the holder of the cheat, is a different individual who have a sympathetic personality in contrast to Zombie, which could explain his heroic acts earlier and her habit to speak with another person inside from herself. So sort of Jekyll and Mr Hyde situation from my opinion, with Zombie being the dominant personality when activating it power while Yui-chan assist powerless to it actions, but having still a innocent child-like mindset, doens't have distinction between good and evil. Possible outcome is that MC kill her enemy anyway, but just "Zombie" by any means, and will potentially spare Yui-chan, no longer having her power.
To be see in next chapter...