As Phlox has pointed out this is not "something their government is doing" (not that it matters as every citizen is responsible for the actions of his government in a democracy) but a very deeply rooted behaviour which enables a peaceful coexistence between two species.Plus you haven't proven the Valakians supposed mistreatment of the Menk was that harsh and even if it was, why should innocent women and children die because of something their government is doing?
When viewing the issue not from Phlox' biological but from a human ethical standpoint, the very standpoint which drove Archer to help the Valakians, you cannot really arrive at a "slaveholding is OK as long as the slaveholder is nice to his slaves" stance.