yeah but like if oda wants to do that
show his crew fighting competent people
....
give killer a much stronger opponent than hawkins . Big moms crew might have been perfect for that. basically both of those crews were relegated to fodder control. That aside from the strength of those two in comparison to other emperors individually.
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.