The clue is why he shot 5 of the victims in the face. Normally to kill someone you would aim for the heart. A psycholgist might say he was trying to erase an event in his life where he was extremely embarrassed.
Not many reasons but could be either money, sex or revenge.
Further information is coming out of the news this evening, he seemed to kill certain people first i.e. twin brother, solicitor, then moved on to workmates before going more widespread.
I heard that he spared a mother and her children, but then shot a man standing nearby. It seems that although he obviously was out of his mind, he stopped short of killing children-thank goodness.