Yeah but that is risky to say MAYBE he will be rehabilitated. Even if he doesnt trick them, twenty one years is still very short compared to the amount of years he took away from those people, probably babies growing up without mothers or fathers which can really effect a person negatively. Not to mention any pyschological damage done to the kids if they were old enough to comprehend. And if he is released the police may be able to monitor him, but they cant follow him around all day everyday, and when they are following him that is taking away from other crimes that the police could be out there stopping...10-21 years is way too little of a sentence for mass murder...an 18 year old could mass murder, do the max, be out by the time he is 39, do it again and still be out again by the time he was 60?
You're not thinking rationally and you're thinking in terms suitable for your own country, not mine. There's no risk in incarcerating a person until he's rehabilitated (or not), and how many years he took away from those people is irrelevant. The years he took cannot be repaid by taking the rest of his.
You also overestimate the resources required to watch a person's doings. You don't need to actually follow people physically around the world (which is pretty old-fashioned), monitoring bank statements, telephone calls, other digital communication, etc. could be more than enough. A police force that doesn't have the capability to monitor dangerous individuals is no police force at all.
The vast majority of people who are incarcerated in my country are rehabilitated, and rarely commit crimes again. However, if he was to serve the maximum sentence of 21 years, and he was still considered to be a danger to society, the authorities are fully within their rights to further extend or renew the sentence. I find it hard to believe that you really think that if a prisoner is considered to be a danger to society, they would just let him go. You probably know better.
The possibility of an individual committing new crimes is not adequate justification for murder, by that logic, every prisoner should be summarily executed simply because they
might commit another crime.
You would have us sink to the perpetrator's level, and do to him what he did to others. Murder is considered to be a
wrong act, a punishable offense, and you would have us commit the same
wrong act, the same punishable offense, to punish the original offense. This is completely illogical, barbaric and uncivilized. Such a notion belongs in ages past, and subscribing to such a notion would be nothing but moral debasement and is the result of emotionally fueled bloodlust.