Witnesses found MacGregor lying face down on the living room floor, surrounded by fragments of broken glass bottles.
Det Sgt David White told the court: “It looks like her mouth has been severed completely.
“Her face more or less was hanging off.”
The court heard there was blood in the living room, including on the ceiling.
White added: “This was one that stood out for me and will stay with me forever.”
MacGregor was pronounced dead at the scene at 10:15.
After the attack, Brennan got on a train to Camelon, in the Falkirk area, where he visited the home of friend Robert McCallum.
McCallum’s then partner Tina Marie Hatton, 46, told jurors Brennan had “blood all over him” and she cleaned him up.
When she asked what had happened Brennan replied he “couldn’t stop” and he was “done”.
Hatton added he had injuries to his hands and above his eyebrow.
The blood stained jumper was seized from a washing machine and the trousers were recovered from the clothing bin.
The blood on the garments was found to have come from MacGregor.
