This is a bit off topic, but I'd agree that John is, indeed, sane. As far as I know, sociopaths generally are sane by most standards: they do not have delusions or compulsions, for example. On the other hand, that doesn't mean they're not dangerous, in the same way that an enemy soldier or a paperclip maximizer is dangerous. Similarly, people with bipolar disorder are usually sane, but sometimes need hospitalization anyway.

I'd tell John that he's not being held because he's crazy, he's being held because he's a danger to himself or others, and he's not leaving until that's changed.