An ideology is a compact and universal. It simplifies and organizes: in this sense it is like a theory. But it can be like old wives tales, or rules of thumb. It is not, therefore a theory. It is a set of answers deployable in many situations: a short cut to action.
An Ideologue is someone who takes their ideology seriously: for whom there are no doubts about the ideology, no need to test it, to confirm it, and, importantly, a person who sees the world through their ideology.
The ideologue is a politician in search of an electorate, an orator in search of an audience, a person who seeks not to understand and share this understanding or to convince with honest argument, but who seeks to convert: to convert others to their ideology. It has become a viral meme in their minds, and perverts their intellect, whatever it might be, to the cause of propagating the ideology.
What’s bad about that? Well, first of all, if you do not know the weaknesses or failing of their ideology, they will not tell you. If they can trick you into liking the ideology for reasons that are inconsistent even with their ideology they will. They care not for you, or your welfare, or even for their own, but for the welfare of their ideology. The world is in a terrible state when ideologues rule.