Demon Possessed or Mental Illness?

I read a news recently in The Star, what intrigue me about this news is the weirdness of the case. It's about a murder case that happen two years ago. Here how it goes:

A couple have a son who was previously warded for a few month for his mental illness. He claimed that he keeps hearing various voices in his head and all this make him unable to sleep and feeling afraid all the time. A few days before the murder, the couple received a call from their son asking them for their help to seek treatment. They then brought their son to see a "bomoh" for treatment. They were on their way to seek the "bomoh" when their son suggested that they stop by at a mosque to pray. While the father was reciting the last verses of prayer, his son walked slowly to him from behind, his left hand on his father's shoulder, while his right hand holding a knife and calmly, he slit his father's neck. He wasn't angry or any kind, just calm. The father stood up abruptly before dropping on the floor with blood spurting from his neck. He then walked away, only to walk back again and slitting his father's throat while saying, "He is misguided" repeatedly. THE END.

So what say you? Is this a case of schizophrenia gone awfully bad? Or do you think he was possessed by a demon? According to Wikipedia Schizophrenia is most commonly manifests as auditory hallucinations (hence the case of hearing voices in his head), paranoid (feeling scared all the time without apparent reason) or bizarre delusions (maybe seeing things that doesn't exist), or disorganized speech and thinking (repeatedly saying things that doesn't make sense) with significant social or occupational dysfunction. Although I won't dismiss the idea that this person could possibly be possessed by demon. The fact that he walked calmly to his father and sawing his neck with a knife while his father was praying, maybe the devil did took advantage of his weaknesses and sickness and overcome him. So I guess it all depends on what you believe in - Science or Spirit. FYI, the guy is currently on trial for the murder of his own father.