Ignoring the implicit racism in your first sentence, and taking 'mitigation' in your second sentence to mean the legal concept of a 'mitigating factor', I disagree with your point, as does the law; Mr Kangas' state of mind does matter.  

First degree murder does require premeditation.  There was clearly no premeditation.  This young man will spend the rest of his life behind bars regardless of how the trial goes--but alleging premeditation is a pale too far.