In my view to kill someone as a punishment for murder is an "eye for an eye" mentality and not part of Buddhist teachings or practice. If someone killed a relative, what possible benefit, as a practitioner,would there be in seeing that person killed too ? None.
From a Buddhist point of view it's most probably correct to say that war is a mark of failure.
The sad thing is we allow such ...... to dictate our views. We must be really stupid to allow this. Wake up,take control of your own life.