Villeins and Slaves

As I work on my research paper, I have noticed that some historical authors, especially ones from before the mid-twentieth century, tend to define the role of native peoples under crusader rule as being one of "villeins and slaves." Some even state "villains and slaves." At first I thought villein meant something similar to the modern word "villain." I was surprised, however, to learn that a villein was a type of serf, the most numerous type of serf to be exact. Aside from freemen, villeins had the most rights of any serf group. It was only when former serfs began to migrate to the growing urban centers of Europe that the term "villain" received its modern connotation. Also, villeinage had existed centuries before the crusades, indicating that it was not some new system solely designed to oppress the Muslims and other groups living in Palestine. Most of the sources that write the phrase as "villains and slaves" are openly anti-crusader and some are even anti-Christian (http://www.ftarchives.net/foote/crimes/c9.htm). I suppose it just shows how cautious one must be in taking sources' impartiality for granted.