It's just Oda's way to say: "Look, in case you didn't get it, you have to look at these 2 as Pirates of that status, cause even if not right now they'll get to that point in the vert following events of this arc"
Killer is a high-Commander, he brutalized Hawkins
The real issue is that Law has no territories under him, neither the interest in taking them or the basis to say he might already have some. He's a character a-la-Mihawk.
But anyway, Oda is (probably) making him an Emperor cause that's his standing. One of the 5 strongest Pirates in the World.