Shirotani is like a wild Pokemon that Kurose had a hard time catching. Used razzberries and ultras, but he would run away. Finally after many tries the razzberries and ultras were super effective and he finally caught him (recent chapters). However Shiro is an unstable Pokemon, and now that biach is back and can cause him to run away... Sorry I play too much Pokemon Go haha :P
I haven't read much of this Sensei's work, but I knew and felt that the one did that to the front door and calling him like crazy would be him. I mean you can sort of predict he would be a crazy character after the first time he came to Shou's apartment... Idk it's not like I haven't read worse Yaoi's, it's just this story is creepy and disturbing because it's really showing a psycho fooling someone and getting away with it.
So most likely the reason Kurose wanted to help Shirotani was that Shiro reminded Kuro of Nishigaki.
SPOILER:
He felt guilty for Nishigaki apparently committing suicide and did not leave Shiro alone in the beginning because he wanted to really help him not be that way. But as time went by he really fell for Shiro. It still doesn't explain his sadistic sort of side but I guess Shiro just makes him feel that way?
I think his "sadistic side" is a side effect of his childhood. He wasn't "special" for his parents, and so now that he is for someone, he's a to possessive sometimes xDDD
In a way, even if he wasn"t in love with Nigisaki, he acted a bit like this with him too.









Oh my gosh the spoilers! Like hell yes that happened! I almost had a heart attack and could not contain myself lol