I propose a fair formula: You are justified in killing based on the temporal proximity of others killing you. If you face a risk of getting killed within days or weeks or months, no chance of having a family, you are more justified in killing to get out of the slavery. If you face the prospect of an early death like 10-20 years earlier than would be normal but will still live for decades and can still have children in the meantime, you are less justified in killing to get out of the slavery. If the slavers are denying you the opportunity to raise your own children, you are more justified in killing their children if necessary to get out of the slavery