Or is this another example of applying modern ideas to people who lived millennia ago?

If it's the first, it's obvious that words change. "Passion" originally meant suffering, as we see from "Passion of the Christ", but now it means intense feelings of desire. "Ward" went from meaning someone protecting to meaning someone protected, as in a "ward of the State". Other words changed just from ignorance. "A nadder" became "an adder", thus losing its link with old English naedre, or "serpent".

If it's the second, you're ignoring the entire concept of slavery from antiquity. Prior to New World slavery, slaves were largely the by-product of wars and debts. In the ancient world, you were supposed to stand by your word and your integrity. If you were going to be captured and you wished to retain your honor, you killed yourself. If your fear of death was greater than your honor, you deserved to be enslaved because you had no integrity. If you were financially ruined, you could retain your integrity by killing yourself. If your fear of death was greater, then you could sell yourself into slavery to cancel your debts. You had in either case proven that you were not a person of quality for whom ideas and principles mattered. This was the mindset.

Of course, with the onset of Christianity different notions came into play. One could show one's devotion to God by calling oneself a "slave of Christ" (δουλος Χριστου or servus Christi as we see from the Pauline letters and Church Fathers). The Christianization of the Roman Empire happened at a time when central authority was breaking down, moreover. The Old Testament notions of kingship were drawn upon liberally as barbarian groups supplanted the Emperor and demanded tribute in what was little different from a protection racket. To clothe this over in respectability, the king was recognized as a temporal version of God, and the simple farmers pledged their loyalty to him in language that was similar to the religious language of the day. Serfs were thus the people who could not defend themselves and relied upon the nobility to fight for them. This is how the medieval society developed - the peasants worked the land, the nobles protected it and the clergy prayed for it. Of course, because the nobles trained for war, had strongholds, weapons and armor, they were able to exert their will beyond the bounds of what was proper, which led to tensions, riots, rebellions, uprisings and other unrest. However, the notion remained from antiquity as did, in the former Empire, language that referred to the villani who worked on the huge latifundiae of the ultra-rich of the late Roman world.

The moral notions survived from antiquity as well - the idea that a serf had no honor or integrity, for example, or the idea that he could not be trusted at his word. Those people who became trusted were raised above the common villains in various ways, either elevated to the clergy or given various licenses (such as for lawyers or doctors).

So is it any wonder that the word continued to denote people who were untrustworthy?