Responses
Actually it wasn't confirmed or anything hat Nnishigaki committed suicide, I think the translations said he just disappears without a trace and Kuro thought it was his fault he left and was sad :( I don't know though we just have to wait for scanlators
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.
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?