In Chapter IV, Locke looks at the issue of slavery and seeks to evaluate it from the perspective of a reasonable judgment about what preserves one individually and human beings generally.
Each person is born free of absolute external authority, that is, we are each our own master and cannot give up this authority except by forfeiting our self-preservation and life.
If self-preservation were not connected to liberty, then placing yourself into another’s absolute power might not always be wrong. Slavery for Locke can really only take the form of punishment – if you have done something worthy of death, the one who by right can kill you, can by right delay punishment and use you for his own service.