The Moon landing, Kennedy assassination, 9/11, chemtrail, new world order, holocaust denialist and other such conspiracy theorists are deluded, idiots or charlatans, and essentially religious zealots. Many of them are assholes (especially eg. the holocaust denialists), but in the end their delusions are more or less harmless. Sure, deluding people into believing falsities is a kind of harm in itself, but the repercussions of these false beliefs are usually relatively small. At most they can cause people to annoy other people, disturb public peace, spam forums, and other such minor things, but not much else.
However, there are two conspiracy theories that are actually very dangerous. We are not talking just about deluding people or making them annoy other people. We are talking about people dying needlessly. We are talking about these conspiracy theories actually killing people who wouldn't need to die otherwise. These are actually dangerous conspiracy theories. They are the AIDS denialist and the anti-vaccination movements.
These are not just some fringe movements that have little to no consequence on people's lives. Many people are buying this stuff and dying as a consequence. For example, if you go to the major AIDS denialist forums out there and follow people there for some time, you will see in real time how many of them are dying. Every now and then you will see someone who has been convinced to go off their HIV medication and who starts getting worse and worse as the weeks pass, until they end up in a hospital, after which it's often too late. Some of these people even post photos of themselves in those forums.
But it goes beyond that. The president of South Africa Thabo Mbeki was convinced by AIDS denialists to ban all HIV medication, which resulted in over 300 thousand premature deaths. This makes it one of the deadliest cults in existence.
The problem with this cult is that not only does it cause already-infected people to die prematurely, it also causes non-infected people to get infected. HIV medication lessens the risk of HIV-positive mothers infecting their unborn children. Many HIV-positive AIDS denialists also engage in unprotected sex with non-infected people, often without informing them. This is a really dangerous, outright criminal ideology.
The anti-vaccination movement has similar traits. Just for some specific numbers, see the anti-vaccine body count website.