[animal]

Usually, there is no moral duty to retreat, but moral imperatives to protect can sometimes create circumstances where retreat is the correct moral action.
The most obvious example comes into play when engagement in the battle would result in you losing. If you have good people and/or good works that are dependent on your continued existence, your primary moral obligation can be to them. Another that comes to mind is when your right to remain in place is removed. If, while facing an aggressor on private property, the owner asks you to leave, you become obligated to attempt retreat out of protecting his right to property.

Life, like money or any resource is not meant to be hoarded, but to be used and spent to promote good. Life is simply our time that we have to be "in action" in the world around us.
There is no moral duty to protect the life of an aggressor. His life is his property, and he is spending it in the way he sees fit.
There is also no moral duty to protect you own life (and the lives of innocents) aside from the good that can be done with it : in the present and future. It is the good that we are obligated to protect. All else is property that we each control as our own for a limited time, and that includes life.

We do have an additional choice that is outside of moral duties, but I believe laudable. The principle of forgiveness can allow us to walk away from a fight if the aggressor allows it or if we have achieved the strength necessary to make his strength irrelevant. Forgiveness must never be codified into law or forced upon someone else because the ability to forgive is dependent upon the relative strengths of opponents. Imposing it as a duty, leads to tyranny over the weak.