Black and Zamasu were gods that wanted to destroy mortality because of its sins. Trunks is their enemy and Black has been trying to fight him for such a long time. Zamasu doesn't know if he should trust Black, he doesn't know who he should trust, he then asks Rosy for assistant. She said "Darkness can't always be bad, there's light in it." Those words changed Zamasu's thinking and feelings for Black.
