Gun Control & RKBA

Your comment:

"Although from what I have read it is unclear if he was even aware the police had arrived "

Speculation as the only person who can answer that is now dead. But even it was verified that he was unaware that his wife called 911, than that points to lack of preparation and training.

"When police arrive on a scene and encounter a person with a gun in their hand is that sufficient justification to open fire? "

As you say, mistakes can be made and it does happen so that's why many strongly advise not to have a weapon in hand when the police do arrive. Your question will not make anyone bullet proof or bring them back from the dead.

"Do the officers who fired have any responsibility or is this simply a tragic mistake with the fault at the feet of the homeowner? "

That's for the inquiry to determine but it can be reasonably assumed that the homeowner would be alive today had he followed basic home defense procedures and had that training so ingrained that it'd be second nature.