What would you suggest he did after the two shots were fired? You obviously think that after two shots he would realise that police would suspect murder. Given that he was the only member of the family who had a motive - he'd be the first suspect. If he went home and did nothing, he couldn't be sure they wouldn't work it out. He had no choice but to go ahead with the call, or maybe that's when he decided he needed the call as an alibi. Either way, he needed to put himself in another location and make the police believe that it all kicked off when he wasn't there.
Not that I would suggest anything; but if you have some knowledge of different murder cases, which I assume you do, you have to admit that a common thing for killers to do, is stage break ins and burglaries. Mostly they do a lousy job of it, but it is very common.
As I said, this is the only case I have heard of where the killer stages a two shot murder as suicide. I would think that would be the LAST thing you´d do!