Here's the story. I get it. I really do. I understand it's frustrating to have someone killed in the line of duty, especially under friendly fire. If he's to be believed, the copy meant to pull his taser and pulled his gun instead. If that's true, can you imagine the guilt he would feel as a result? He may, or may not have meant to pull his taser. We will most likely never know.
What I do know is that if this kid had not been in a fight, and had not been resisting arrest, there wouldn't have even been a reason for the cops to be there, let alone use a taser, and mistakenly (or not) a gun. If the cop had walked into the station and shot a man lying on the platform who had no other reason to be there, this would be a tragedy. As it stands, why was he resisting arrest? When a cop says you are under arrest, you comply. You don't rebel and add another charge to the books. Especially if you have not done anything wrong.
Simply by resisting arrest he put his life in danger. First the fight, then resisting arrest. Everything else that happened snowballed from there.