“Servants” or “Slaves” in Romans 6:17 et al.?

“δουλους” could mean “slaves” or “servants” depending on context (Thayer's Greek-English Lexicon). In the context of a Christian's relationship towards God, “servants” seems appropriate since believers become δουλους by acceptance. The voluntary aspect indicates servanthood rather than slavehood. The NASB also translates “δουλους” as “servants” in Revelation 10:7.  The ESV 2011 Update has changed "slave" to "bondservant" in 1 Corinthians 7:21 and elsewhere (Link to the List of Changes).  The following explanation of the move from "slave" to "bondservant" is explained by the ESV translators in the new Preface (Link to the Preface):

Third, a particular difficulty is presented when words in biblical Hebrew and Greek refer to ancient practices and institutions that do not correspond directly to those in the modern world. Such is the case in the translation of ‘ebed (Hebrew) and doulos (Greek), terms which are often rendered “slave.” These terms, however, actually cover a range of relationships that require a range of renderings—either “slave,” “bondservant,” or “servant”—depending on the context. Further, the word “slave” currently carries associations with the often brutal and dehumanizing institution of slavery in nineteenth-century America. For this reason, the ESV translation of the words ‘ebed and doulos has be

The ESV translators do not agree with the KJV translators in every instance of translating "δουλους", but they nonetheless recognize that the context determines what the word should be in English.  It is an error to say that all translations of "δουλους" as "servants" is wrong.  As stated earlier, the context of Christian soteriology justifies translating "δουλους" as "servants" in almost every instance.