How come Murder could be the punishment for Murder?

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According to the law system, Death penalty is the capital punishment for a Murder in many countries. But now a days it reduced as well in India. Since in many situations court is supposed to give capital punishment to criminals. Do you think is it fair?? Depending up on the cruelty of case, court is giving this capital punishment. But in my view, It is totally wrong ! It is really Injustice, One should get punishment for their mistake, Punishments are being given not in the motive of Revenge. Punishments are just to realize their mistakes.. If law system also acts as a normal human being, then what is the difference between human emotions and court? Death penalty is looks like a revenging against the one who commit mistakes?? Why this revenge could be made in the court? Can any one revenge anybody whom they would like to revenge?? What are your views??