[Forbin]

This part.

Spoiler:

But something happend that got Lucy angry or jealous and she ended up killing Kouta's sister

I added onto that with a new speculation

Spoiler:

She didn't kill what she is already. And that means they can infect people. And then Robocop must be infected, and that means him and Nana are going to get it on.

Phew now I understand why you are confused, boy did I go out to left field to make up that Theory.