These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Angst? What Angst?: Setsuna's life was so awful at the start of the series that he doesn't mind the prospect of having his heart eaten in one year by an Oni. In fact he is grateful that he is given the opportunity to at least not be a burden to his sister and give her the money she could use to start a new life that he believes he messed up due to her having to support him.
Epileptic Tree: Many fans assume that before Setsuna's heart is eaten he will get a transplant and be romantically involved with one of the Oni Princesses.
Last chapter subverts the first where instead the Zombie Clan uses a technique to allow him to live and keep the healing qualities of the heart, and the second three of the Oni Princesses and Armia are after him.