Insanity plea doesn't really work like that. You can have insane reasons for murdering someone- voices told me to do it, they were possessed, ect. But the insanity plea is really more for people who cannot understand what they have done, as well as understand the punishment for their crime.

A killer already is insane by society's standard. But for them to be insane by the court's standard, they have to lack the capacity to grasp what they have done. If a grown man with severe autism pushes a person down the stairs and they die in the process, the man may very well not know what he did. To punish him for it would not make sense to the man with severe autism because he would be unable to make the connection that pushing the person led to their death, for which they are required to be punished.

The man would simply sit there in prison not understanding why he is in prison.

The man in the article, however, seems to have had full awareness of the situation. He is able to grasp that he killed his girlfriend. He would most likely be able to grasp that, while he himself may not agree with it, the law stated that he will be put in prison or even on death row for killing her. He has the ability to grasp that most likely.