He said Browne abused other guests and got into numerous scuffles before Stewart came to restrain her.
Witnesses described how Stewart dominated, intimidated and made Browne scared, telling her "You f*** disrespectful b***, my nieces are f*** here".
He said Browne's disproportionate response was to grab a knife and stab Stewart.
This made her guilty of murder under both its legal definitions, he said: she not only intended to kill when she plunged the knife 11cm into Stewart's face, but she was reckless to the dangers of her actions.
Webby described how, out of all the utensils in the kitchen, Browne chose the largest knife before approaching Stewart with it behind her back to maintain the element of surprise and then striking at her head - a vulnerable target.
"It was a carefully directed blow," he said.
"Can you really infer any other intention but an intention to kill?"
Webby said medical experts had testified during the trial Browne suffered antisocial and attention deficit disorders.
He said even the defence team's own medical expert had said during the trial that attention deficit disorder combined with alcohol consumption had a direct relationship to Browne's violence and emotional responses.
These responses were "often out of all proportion to the triggering event", he said.
Browne's defence lawyers are due to give their closing argument today.