This is a difficult question. I have no idea what drives irrational beliefs. Here are some conjectures:
* Perhaps the AIDS deniers are racist. Most of the lives lost to this nonsense were in Africa. The president of South Africa, Thabo Mbeki, was for years influenced by the AIDS deniers before adopting the scientific view. Maybe the AIDS deniers want to maximize the death toll in Africa.
* Perhaps the AIDS deniers are homophobic. Their policies may be crafted to kill as many homosexuals as possible.
* Perhaps the AIDS deniers are religious fundamentalists. They may believe that AIDS is God's just punishment for recreational drug use, homosexual intercourse, illicit sex, etc., and scientists should not intervene in God's plan. * Perhaps the AIDS deniers are hoaxers. They don't really believe what they preach; they just want to see how many people they can fool, how many scientists they can irritate, and how many times they can get their names on the web.
* Perhaps the AIDS deniers are eugenicists. They may be trying to improve humanity by ridding the planet of the most gullible people, the very folks who take their advice.
AIDS Is a syndrome and not an illness, which strictly means that it is not 'caused' by a virus but is a manifestation concurrent to the virus (syn-drome from the Greek: to run along with, as con-current in Latin: to run along with). That's why I don't like the accepted phrase that hiv causes aids - hiv is a virus, when it is present with a specific illness the two together are called aids.
Strangely AIDS is defined differently in the US and in Europe.
Those are the details, the lack of clarity in the details that annoy me about both denialists and people who recite 'hiv causes aids.' The former are quick to ignore that people are dying of something, while the shortcut of the latter is too simplistic.