I guess there could be outright premeditated cold-blooded murders that are disguised as suicides, but I can't imagine that's very common. I mean, you can't bring a body in with a zillion bullet holes and tell the coroner it was suicide. And, as you point out, a noose is going to (hopefully) generate a lot of suspicion and a thorough investigation.
The main area of uncertainty is not the number of police killings, but the number of justified versus unjustified killings.