
It's a common trope for zombie movies to have someone sacrifice themselves for the rest to survive (ex. Train to busan), it shows how deeply they cared for each other.
Also, from a storytelling perspective it made sense that Caleb was the one that died. He did too many bad thing through out his life ( like killing unarmed survivors), so his death for his master was an atonement for the sins he had committed.
Not all stories need to have a happy ending to be satisfactory.

To the people complaining about how rape was treated.
To me it seems like a parody of the "I love you so I will rape you" trope. Reo obviously loves him, so he rapes him (something often happening in older yaoi) without considering his feelings and then assumes they are dating because the uke is too scared to go against him (which he is completely oblivious of and probably thinks it was consensual too).

It felt like I was reading a different manga halfway through.
It started as a college playboy getting tricked by psycho/stalker loner with ulterior motives but after chapter 2 it felt like I was reading a drama about office workers with a sadistic seme.
Also the uke's personality changed so drastically that he didn't feel like the same character.
http://www.mangago.zone/read-manga/moto_yakuza_ga_bl_sakka_ni_nattarashii/uu/d_chapter-1/39/
Wtf is this. Best redrawing I have seen in months. XD