Even among the family members of accountant Stephen Pasceri, the belief is that he killed Dr. Michael Davidson, the physician of his mother who died on November 15, 2014, as his vengeance.

However, Daily Mail reports that one possible reason goes beyond the death of Marguerite Pasceri. It could be the $8,000 medical bill left for Marguerite to pay after her husband Greg died of heart attack in December 2011, although the family did not question the quality of care that Greg got at Massachusetts General Hospital.

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Pasceri wrote to Sen. John Kerry and Rep. James McGovern to seek an investigation behind the enormous medical bill, reports the Worcester Telegram.

He shot Davidson, a cardiovascular surgeon, twice on Tuesday inside the heart room and then used the gun on himself in another room at Brigham and Women's Hospital in Boston.

Davidson had sensed danger when he saw Pasceri that he ordered everyone in the room to get out immediately.

Davidson's wife, a plastic surgeon, is seven months pregnant with their fourth child. They have two girls aged 9 and 7, and a son aged 2, according to Davidson's father-in-law.

Other doctors said that Davidson was a very kind person, which surprised them why Pasceri killed the cardiovascular surgeon.

"You all should be absolutely assured that Michael Davidson was one of the kindest, best possible men that ever walked on this earth. We know that and his patients knew that and his family knows that," Dr. Andrew Eisenhauer shared with ABC.

Even Pasceri's sister, Marguerite Jolym, is puzzled by her brother's actions. She told Boston Herald, "I think it comes down to the fact that my brother thought it was the doctor's fault that my mother died. I don't know why my brother blamed him. I really don't know why."

TagsBrigham and Women's Hospital, shooting