Well 19-year-old her, apparently under the influence of Charles Manson and the rest of the family, committed a crime which in many Western European countries would have seen her released in under 20 years.
It's very easy to paint people as monsters and/or as being pure evil because it sells copy and keeps people titillated and a little bit frightened. That fails to take into account the circumstances or how someone might change over the course of 50 years.
IMO there are three elements to incarceration, punishment (for the crimes committed), public safety (keeping a dangerous person away from the public and whatever deterrent effect harsh sentences may have) and rehabilitation (getting an offender to become a net contributor to society).
Effective penal systems (where effectiveness is measured in terms of low levels or recidivism and low overall crime) tend to focus primarily on rehabilitation. The US system, with long sentences and harsh prison regimes, seems to be more about punishment. I do however question whether a Scandinavian prison system would be compatible with US society. Here in the UK, attempts to focus more on rehabilitation have been successful in terms of reducing reoffending but have been unpopular with large sections of the public and press (who seem want to see prisoners doing hard labour on a bread water diet) - and so those initiatives have withered on the vine.
I cannot comment on van Houten personally, having never been present in her parole hearing(s) and having knowledge of her time in prison beyond glancing at a couple of headlines but it seems to me that she is exactly the sort of person who should be released from prison, a model prisoner who presents little or no threat.