What you said about Goku's killing intent is true, however that does not change the fact that Black counters Goku heavily.
Future Zamasu basically explained his powers, saying that if he kills Goku too quickly, he'll lose the source of Black gaining even more power, and told him to take it slowly while killing him. But yes, had Goku gone with killing intent when he went on a rampage, he might've won this battle, but he didn't.