To characterize this killing as being only a "mistake" is absolutely ridiculous.   To shoot at an unidentified person, without having any proof that they pose any threat, is more than being just irresponsible.   It is criminal negligence.  

This man well deserves to be harshly condemned by society.   Taking the life of another human being is also an affront to God, and a violation of his holy commandments.   He will thus need to do much to atone for this awful sin, as God's judgement will be what he needs to fear the most.