HIV is very effective actually, long incubation, easiness of spreading, perfect for long term killings. Smallpox works much faster, and is much easier to diagnose, you could theoretically modify HIV virus to do more damage over time, and work slightly faster, and spread over air and not it's standard way, that way you could kill a lot of people in a much faster way.
Over a much bigger area as well.
If HIV was designed to be a weapon of population control, it did serve it's purpose in the early days of it's discovery, but now there are ways to protect yourselves from it.
It's still an effective killer though.