The purpose is to prosecute him for the crime he has been charged with. That charge is Involuntary manslaughter, which explicitly means he was negligent in a way that lead to the death of another person. The purpose of having him(or literally anyone) stand trial in the American justice system is (supposed) to examine the evidence and determine if it supports the charges against the (presumed innocent) defendant. You don’t get let off the hook for your mistakes just because you ‘feel absolutely terrible’ about it. Your understanding of fault in this situation is incorrect.