(1) Traditional Catholic: Albigensians were evil and vicious.
(2) Traditional Protestant: The Albigensians disobeyed the Pope, so they must have been some kind of proto-Protestants and therefore really good guys.
(3) Postmodern revisionist: The Albigensians rejected the hegemonic power structures of their day, so they must have been really good guys.
The primary sources are spotty - they left little behind in the way of testimony on their own behalf - partially because they were doomsdayers who didn't see the point in scribbling while the world was ending, partially because the Church burned their recruiting materials and partially because the Provencal language they spoke died out and any remaining documents that were left behind were probably unintelligible within a couple of generations and were thrown away or overwritten.
There are sources from Churchmen which divide roughly into two groups: clergymen who worked for the King of France, who had a vested interest in putting the Albigensians in as bad a light as possible - such as accusing them of human sacrifice, cannibalism and sorcery; and clergymen who were engaged in debating them and trying to convert them - these are the ones who delved more deeply into their theology and beliefs.
There are a few scholarly articles that examine the later sources - clergymen who tried to familiarize themselves with Albigensian doctrine and practice - and try to read in b etween the lines to get a clearer picture.
I'll try and find the articles' titles and authors.
Basically their theology was an interesting mix of two older heresies which seem to have been imported from the Near East: Manichaeanism and Marcionism.
From Manichaeanism they got the concept of having a ruling class of mystics who were kind of like what we call gurus. They also adopted the idea that physical matter was evil and that eating and sex, even marital intercourse, was sinful.
From Marcionism they got the idea that the God of the Old Testament was the Devil and that he created the physical universe. They adopted Marcion's practice of removing large parts of the Bible, including the Old Testament and many Gospel passages as demonic additions to the Scriptures.
They had one sacrament called the consolamentum - it was given to those who were perfecti, those who had successfully starved themselves. They believed that property ownership was carnal and therefore evil. They rejected social distinctions and were genrally very collectivistic.