I was just wondering this question from Book One of Augustine's On Free Choice of the Will
Nothing directly but implicitly - obey.
There is extensive discussion in Free Choice of the Will of masters slain by their slaves (I.4.9.25 ff.) I think Augustine indirectly follows St Paul on this subject. The slave could indeed kill her or his master but that is not the way of Christian spirituality. The following extract from Margaret Mary might help. Her language is that religiously engaged - not what a philosophical reader nowadays generallly encounters - but the ethical point is easily extractable:
Like every good ancient scholar, he [Augustine] gives us the etymology of the word ['slave'], stating that prisoners of war, instead of being killed were preserved by their conquerors, and were called servi from that fact. For by nature and as God first created us, no one is the slave, either of man or of sin. But servitude is penal, and is the sanction of the natural law. If nothing had been done in violation of that law, there would have been nothing to restrain by penal servitude. Since this is so, Saint Paul urges slaves to submit to their masters and serve them heartily and with good will, so that if they cannot be freed by their masters, they may themselves make their slavery in some sort free, by serving, not in fear, but in love, until all unrighteousness pass away, and God be all in all. ( Margaret Mary, 'Slavery in the Writings of St. Augustine', The Classical Journal, Vol. 49, No. 8 (May, 1954), pp. 363-369: 363-4.)
The text in the background here is St Paul's 'Letter to Philemon', addressed to the owner of a runaway slave, Onesimus. Paul directs Onesimus to return to his master, Philemon, and asks his Philemon to be reconciled with the slave. Even as a slave Onesimus will have the freedom of being obeying willingly as Philemon's brother in the Lord. Slavery dos no harm to his spritual life and development.
This answer leads us into theology but you raise a genuinely ethical issue in asking why a slave should obey rather than kill her or his master. The issue of freedom-within-slavery arises more familiarly to philosophers in the (non-Christian) writings of Epictetus.