If its murder does that mean you can then do whatever you want?
Eye for an eye is just another specious premise like money is the root of all evil.
If I rape your daughter do you get to rape mine? Do you get to rape me?
A better system is to work out what rights can be objectively defined for a human and then work a justice system based around that.
I can make a strong case for voluntary interaction and self defense, whats the case for eye for an eye? Not to mention in many cases it would invalidate rights you don't have the right to.
Logically mirroring a situation doesn't make your choices logical.
You never have the right to end another persons life against their will. Now you have a right to kill someone to defend your life, but thats because your rights overlap them when they decide to act or threaten to kill you. Your freedoms end where anothers begin, and the freedom not to be murdered is innate to all humans no matter how they act, making the distinction between murder and self defense.
Under the eye for an eye system, do I get to kill you if you kill my brother in a car accident you were responsible for?
It's not logically congruent to OK murder just because death is involved in the situation.
What right is being infringed by not being able to murder someone who murders someone you know? You have the right to be made whole, but how can that right invalidate anothers right to life?
If someone takes your life then there is a massive debt owed to you, and to that extent anyone you designated your post humous will to, but that debt does not encompass their own life unless they agree to it. Its their right and they keep it for so long as they do not make it necessary for another to kill in self defense.
Morals are objective constants, or they do not exist at all. Murder does not suddenly become moral just because something bad happened.
You can argue that a murderer should have to repay the monetary value of the life, or pay in the freedoms and time that they took, but how can you ever justify killing someone when it is not to defend your own life?
By logical extension, in some instances, theft could warrant the death of the thief, say if the thief stole money needed for medicines/food that then lead someone to die. They were responsible for anothers death, now their life is up for grabs.
Even if an objectively definable human morality does not exist, I believe it to be the best choice to base any system of morals that we make up to have objective criteria that are consistent with the nature of man and personal freedom.