Epidemic disease, along with famine and war, make up the Big Three causes of population decrease. Generally speaking, disease is by far the most effective one. So, it would be natural when thinking of "culling the herd" to think of disease as the culprit.

I get the idea that if he'd said "bubonic plague" or "dengue fever" instead of AIDS your reaction would have been far less horrified, not because those diseases are less horrific in their effects on the body, but because AIDS is associated in most minds with a specific minority group which is widely persecuted in the world, and this makes it appear that Colin was wishing death specifically on that group, not others.

I doubt this is where his mind was at. By 1989, when that book was published, anyone who kept informed knew AIDS was spreading rapidly into the general population and at that time AIDS was seen as nearly 100% fatal. It would have been the obvious candidate as the agent for the next human die-off. I'm sure he wasn't thinking about gays as the exclusive target of the disease; they were just the unfortunate vanguard in coping with what is a pan-human disease. But I can see why you experienced revulsion at the thought that he may have had such an idea in his mind.