He was convicted of manslaughter not murder, so the court accepted he had not intended to kill the child. An article I read about the case stated that he'd been utterly negligent about gun safety, had illegally enhanced the power of the gun, had no gun licence and lied to the police afterwards about the circumstances of the shooting. If it hadn't been for those aggravating factors, I suspect his sentence would have been much lighter.
The whole case was shocking but his failure to apologise to the parents is unbelievable. At the very least you'd have thought he'd be stricken with remorse. Apparently the extended family has been split down the middle with different parties taking different sides.