Responses
If you reread the first chapter with the right translation, when Adam ask details about William, he asks him if he has an older brother. Here he's talking about Mark, who have a similar face to William.
That's the reason he kept William with him, because he looks like Mark. And the zombie they kill is Linus from the second story.
All I can say is "Oooohhh... What the fuck?", like each story was so weird and ended up romantically tragic and I didn't even expect them all to be connected. The ending of Ch5 was actually closed the manga really well, I teared up due to the shock and the realization.. The only story not adding up is Mark/William and Adam's story, like literally so frikin' confusing.
But anyways, this shit is a brain twister, its insane, story wise, and art wise.