A reformed murderer who has done the time for the crime and been rehabilitated has as much right to a life as all of the rest of us in society. A decent justice system based on rehabilitation (we do not have one) would see that he'd not be a significant risk, or he would not be released.
We have a duty to treat everybody fairly, and a duty to look after those who require looking after. That means we need a police system to protect the vulnerable, we need a justice system to make sure that everybody knows that justice will be served if you commit a crime - to make crimes not appealing, and also to rehabilitate those who choose to commit crimes so that they do not repeat their crimes.
Society agrees rules which we will all be subject to in advance of events occurring - part of the reason we agree to look after the disabled because we might become disabled, or our children might be etc. It's simpler with health care - we agree to provide health care free at the point of access because we all want to be able to draw on that should we need to.
why should he have that right, if he just killed someone? the way in which some of these people are killed is just sick in the head, thats probably why they're being put down