Highly unlikely that he would be successful with an insanity defense. That is a very difficult defense to establish, and in most cases, means a showing that you cannot distinguish between right and wrong, OR are so insane that you were mistaken about what you are doing....the classic example being a person who shoots someone in the head believing it is a watermelon, not a head. If a person truly thinks that, and it can be proven, they will be remanded to the custody of the state for those criminally insane (often a far longer sentence than regular prison because, well, they are nuts and need to be locked up).
In this case, it seems he is likely to be quite mentally ill, but also not criminally insane. Those are not inconsistent statements. Both can be true, and in this case, likely are.
We do a very, very poor job with mental illness in this country. And, I believe that over the course of time, we are going to learn that these disorders are far more "physical" than mental. We just don't know enough about the brain (yet) to be able to identify and treat what are likely organic malfunctions of the brain (like Type I diabetes is a result of a mechanical dysfunction of the pancreas).
But, of course, today, we treat these people very poorly as if they can "control" their mental health and if they aren't well, it is surely "their fault." Sad.