...he was showing her how to use his shotgun before she suddenly ran away from him for no apparent reason.
Why, that's completely plausible! What better place for shooting lessons than one's car in the middle of Frankston? She must have run to make him a sandwich because of all that hard work wielding a firearm (they're HEAVY!), but unfortunately she failed to determine what KIND of sandwich he wanted! Seeing as Robinson had threatened her with a gun a week before, it's clear she wasn't learning her lesson about the sandwiches!
And in a sterling display of the quality of his character, Robinson refers to the murder victim with such endearing and respectful terms:
He chased after her, he said, in an attempt "to talk to the silly bitch".
Hence, when a neighbour opened the door to her cries for help, THAT's when he SHOT HER IN THE HEAD.
Like how a Justice of the Supreme Court might "accidentally" sentence you to life in prison, scum. Whoops!