"For example, throughout much of history many Christians were slaveowners. Is this in itself a reflection on Christianity -- or does it only reflect on Christianity when slaveowners appeal to Christian teaching to justify the practice of slavery?"

Slavery and the praise of virtuous slaves and servants, is written deeply into the Christian religion. I find it no surprise that it was so easy to use the Bible to justify slavery, since the Bible in fact, does justify slavery - or rather, it does not even question whether or not slavery is a good thing, it accepts slavery as part of the natural order of things. The concepts that there must be a dominant person and a submissive person in every human relationship, i.e., all relationships are hierarchal by nature, is an unexamined assumption throughout much of the Bible. Only in the New Testament do we see any hint of a "kinder gentler" view, and that little neough, verses meant to emphasize all Christians are equal before Christ in Ephesians, I believe - yet this is not meant to supercede the slave/master dichotomy here on earth, only in heaven.