Clearly Ashley was no one's idea of a golden child. And she does bear a lot of the responsibility, particularly as she got older, for her own behaviour. But I think it's pretty clear that there was something seriously messed up with this girl too.
And it wasn't being restrained that did her in. Being restrained looks horrific when you're watching it on video. Of course it looks torturous, someone being restrained is usually kicking and screaming. But it looks far worse than what it is and it's often necessary for safety reasons, both her safety and the safety of others.
What really did her in, I think, was the constant moving around which circumvented the automatic review process that occurs when someone is confined to isolation for a set period of time (can't remember if it's 60 days or 90 days, it's been a while since I've seen/read up on her story). It amounted to shuffling off a problem to someone else, from one institution to another. But Ashley wasn't just their problem, she was also their responsibility. And transferring prisoners is not up to the guards who are restraining her, that order comes from farther up the chain. They are the ones responsible for allowing the situation to degrade to the point that it did.