Yes it was a deliberate firing.
Had he used his taser, there would have been no problem. He didn’t use his taser, and the man died.
He mistook his service firearm for his taser.
He meant to stun the man, and instead killed him.
I can’t defend what he did, because it was a fatal accident. He was clearly in the wrong. It was not a murder.
I did not say it was murder.
I’d probably classify it as manslaughter.