It's impossible to know what the motive is without being told by the perpetrator or the police from what they have ascertained from their investigation. It is pretty clear that there was a motive, it was premeditated and it wasn't a chance happening that escalated or anything like that. He had taken a taxi there (and a large knife), refused to pay and then committed the atrocity and then just went home. People, especially those suffering from mental health issues, can "snap" and do something that they haven't planned but all the released information points towards something pre-meditated and that means a tougher sentence should be sought.