But his lawyers can argue he had a psychotic break which lasted for some weeks.
I witnessed one that lasted for nearly a month straight. After two weeks without medication in the hope he would restore his sleep cycle as he had been sleep deprived for some time and had PTSD, medication was attempted.
Two weeks of anti-pyschotic medication brought him back, as he had a nightmare going on in his head no one could break through. He was able to eat, dress himself, shower, walk around, etc. but responded to nothing done to him until then.
It was a break through and he continues to have to be given prescriptions carefully for over ten years since that time. Even so, he has had serious setbacks from medication changes to prevent liver problems and had psychotic breaks several times seriously assaulting people around him whom he identified as others from the past who had harmed him but were not there, but now is stable in care.
We'll never know what the attorneys or doctors will find, but the example above might have appeared normal if he wasn't in a hospital ward being observed day and night for his behavior. Holmes wasn't in a psych ward. Doctors may find proof of a psychotic break. That may fit the legal definition of insanity for Holmes. It did for the person I described above.
We may not like to accept that is why he did it, and it doesn't excuse the crime, nor am I saying he should go free. But if he is found have been insane, his problem may continue to be so profound he may never be able to walk around without supervision again.
JHMO. Not an attorney. Not a doctor. Just an experience.