Not surprised by his actions. In a country where people are not held responsible for their actions, this man made sure this person paid for his sons lives with his own.
That being said, we can't all just shoot people for using bad judgement. Although in some cases it seems like swift justice. The problem is that the person who was shot is someone's son or someones father. As crapy as it is, 2 wrongs don't make a right. Ultimately the father has created the possibility that his sons lives are paid for with the drunk driver's life and possibly the rest of his own life.
"There is no greater tyranny than that which is perpetrated under the shield of the law and in the name of justice. -Charles de Secondat"