All we REALLY know is that this guy tried to interfere with a flight crew. How do you know if he was mentally ill? You put him in custody and have doctors look at him. Until then, you treat him as the law dictates.
If this guy was a mental facility escapee off his meds, then yes, we could already make a determination that there was no criminal intent. This man has no history of such. Until a couple of days ago, this man was indistinguishable from any other captain on the line. Irrational violence and religious zealotry are not hallmarks of a healthy mind, but they are not mutually exclusive from criminal intent.