He killed him out of rage. He didn't know if the guy had a heart attack or passed out from diabetes. It wasn't determined he was intoxicated until after he was dead. I can understand wanting to hurt someone who just killed your family but what if it truly was an accident due to a medical condition or something else.
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"The said constitution shall never be construed to authorize Congress to prevent the people of the United States who are peaceable citizens from keeping their own arms." - Samuel Adams, August 20, 1789