Although you've tagged this question about The Following, the truth is that this is a much, much broader question which is far more interested in the psychological reasoning behind people joining cults and their blind dedication and loyalty to their leaders.
I've tried to focus on that aspect of the question and to aid in answering it have researched a few online articles. Given the severity of the topic, I've also tried to only use articles from reputable(ish!) sources.
This is by no means a definitive answer and I do believe your question is so broad that it will be difficult to get anything like a single, definitive, objective answer. Additionally, there are so many resources available to answer the question, that it is hard to not automatically be drawn towards though which support your opinion (which is what happened to me in my answer below):
So first things first, what is a cult? From the Oxford Dictionaries:
A system of religious veneration and devotion directed towards a
particular figure or object.
An alternative, and more expansive, definiton is given by the Cult Information Centre:
The Cult Information Centre (CIC) defines a cult as a group having all
of the following five characteristics:
1. It uses psychological coercion to recruit and indoctrinate potential members.
2. It forms an elitist totalitarian society.
3. Its founder leader is self-appointed, dogmatic, messianic, not accountable and has charisma.
4. It believes 'the end justifies the means' in order to solicit funds or recruit people.
5. Its wealth does not benefit its members or society.
In your example, that particular cult is focussed on devotion to Joe Carroll and your next obvious question was why?
Feldman and Johnson  describe some possible reasons:
Members of cults are recruited and controlled by means of what
Hochman describes as the concepts of "miracle, mystery, and
authority." Miracle refers to an ideology that ascribes special
powers to the leaders and to the group's activities. Mystery serves
to obscure the actual be- liefs and goals of the group, such as
power and control, and creates an atmos- phere of awe. Authority
reflects the claims on members' time, talents, lives, and/or
property to meet the needs of the group. Through these concepts the
cult establishes dominance over individual members.
This follows to a degree what Abraham Maslow  proposed in his now iconic "hierarchy of needs":
He argued every person has fundamental needs (represented at the bottom of the pyramid) along with other needs such as the need for self actualisation. Maslow believed that the most basic level of needs had to be met before the person would strongly desire the higher needs.
In other words, using the diagram, he argued that people would fight for basic amenities like food, water and shelter, before longing for safety and security, before longing for love and companionship, before longing for the respect of others and a development of confidence, before longing for real personal enlightenment and development.
Using this structure along with the reasons provided by Feldman and Johnson, it's easy to see why people can be attracted to cults. People are given a false sense of safety, love and bbelonging. This false sense brings with it a fake sense of esteem. It then leads members to a form of self-actualisation, which can never succeed since they are trapped by the previous layers of the pyramid being created on false truths. Only when they break free of this can they truly develop.
In The Following, Joe Carroll met his followers in prison. He had years (nine I think) to slowly convert them to his way of thinking. They were all intrigued with him and he used this to corrupt them and offer them a sense of belonging that they felt they couldn't find anywhere else.
So I think this, coupled with the rest of the articles, provide some indication as to why
people subscribed to his cult and ideas. However, one final question remains. Despite following him, why would people be moved to kill for him?
Zimbardo presents this line of reasoning:
Stanley Milgram (1974) developed an ingenious research procedure to
demonstrate the extent to which situational forces could overwhelm
individual will to resist. He ‘shocked the world’ with his unexpected
finding of extremely high rates of compliance to the demands of an
authority figure to continue shocking an innocent victim to the
maximum possible level (also see Blass, 2004). He found that about
67% of research participants “went all the way" up to the top shock
level of 450 volts in shocking another person that were supposedly
helping. Milgram’s study revealed that ordinary American citizens
could so easily be led to engage in “electrocuting a nice stranger,”
as the Nazis had been led to murder Jews.
So I would argue this is why Joe Carroll's followers are able to go to such apparently astonishing lengths to follow his word. They trust him, he gives them a feeling of belonging and security and he rules them absolutely. This makes them believe they belong with him and as such lets him get away with poor treatment of them. After all, how could they turn away and reject him when, even when treating him badly, he still offers the best hope any of them could have (or so he's led them to believe). Given his power and dominance, they are willing to follow him blindly and have even become killers for him.
Hopefully these articles all provide some food for thought and give good answers to some of your questions. I will repeat though that this is by no means a definitive answer and someone with much more experience than me in the field of psychology would be the person to speak to!