I read 'Killing For Company', and as backed up with the first link, it seems that Nilsen was able to carry on murdering because of a bizarre series of events which prevented his capture early on. Indeed - were it not for his 'backlog' of bodies, hw could have gone on for much longer.
The tragedy of these men's deaths is that some of them remain unidentified and unmourned.
It may not be possible to release Nielsen, I do not believe he has co-operated with any attempts to rehabilitate him, and he may well be viewed as a continuing risk in view of this.