ok so:
since they were kids, tomoya had developed romantic feelings for shu but he knew it was totally a doomed kind of love that's why he made shuu promise one thing—eating shuu, because that's the only way he (tomoya) thought he and shuu can be together.
throughout this manga, it is hinted that everyone (at least the characters we see) gets the intrusive thought to eat someone they love when they die;
it was written by shuu in his etymology report in chapter 1,
zombies in an attempt to regain a soul in chapter 2 (not really confirmed i just thought it was poetic that zombies eat humans to try and love somebody again),
shuuhei confirming that he's not capable of loving anyone, (total bullshit btw) realizing that eating him cannibally is the only way their bodies can be connected in chapter 3,
shuu's girlfriend confessing that she thought of eating her dog when it passed away in chapter 4.
additionally, in combination of both chapter 2 and 3 the story hints at shuuhei's belief that dead bodies don't possess soul. so in the ending, it is plausible that that shuu eats tomoya alive.
Bro i might drop this if shuuhei eat him, thanks for explain it thoroughly
Why he wanna eat him so bad, what's going on