In one of my recent lectures I made the point that insanity is a relative concept. For those experiencing hallucinations and delusions these things are very real. as such they are responding rationally to the information that their senses are supplying to them. I know I have mentioned thisbefore but the movie "A Beautiful Mind" shows just how these hallucinations are real to the perceiver.
As I understand it delusional behaviour is defined as a person acting on perceived images or things that are not part of any consensual "reality". However just because it the perceptions are not consensual does not mean that they are not "real" ..... particularly if reality itself is an inwardly generated illusion.
This is not to say in any way that a person acting in response to these illusions, hallucinations or delusions is not a danger to his or her fellows, far from it (too many cases have taken place were people have been killed by such people) but we need to accept the subjective reality of these perceptions and act(and treat) accordingly.