Screwed Up Justice

Mitchell Johnson, one of the shooters in the Jonesboro, Arkansas, school shooting in 1998 was released from prison last week after serving seven years in the juvenile system. This just seems wrong to me. These kids killed 5 people, and wounded 15 more, in a cold blooded, deliberately planned attack on their own teachers and classmates. Because he was 13 and could only be tried as a juvenile, the state had to release him on his 21st birthday, which was last week.
Now, I understand the reasoning behind the juvenile system and why many of those laws exist, but this is crazy. Not only did he only serve seven years for the murder of multiple people, he also has no criminal record now. I read one comment from an advocate for the juvenile system who says that since the system is aimed at rehabilitation, it is better to release him now than 10 or 15 years down the road when he could not reacclimate himself to society. That argument is all well and good, but who's to say that this kid is rehabilitated? That had absolutely no bearing on his release, he was released because the state required that he be released by a certain date, regardless of what kind of rehabilitative progress, if any, he has made. Now, for all anyone knows this kid could have the exact same tendencies (or compulsions, or sickness, or whatever terminology you wish to use) that drove him to murder seven years ago, but he is a seemingly innocent member of society with no criminal record. Ironically this means that the juvenile system has allowed him to serve an extremely short sentence for a very serious crime, released him back into society with no guarantees of rehabilitation, and given him absolutely no criminal record, enabling him to do whatever he pleases, including buying a gun. How does this serve the best interest of anyone, him or the rest of society?