The debt-ridden dad then shot his wife as she slept in the couple's four-poster bed before torching the family and mansion and turning the gun on himself.
Police have released harrowing details of the night Foster - faced with repossession of his home - embarked on a horrifying killing spree slaughtering his family and pets.
It is thought pretty daughter Kirstie, 15, was chatting to chums on social networking site Bebo when her father blasted her in the back of the head on the third floor of the rambling Shropshire house.
Foster, 50 - who was "blind drunk" after his oil pipe insulation business collapsed - then crept downstairs to the room he shared with his wife and executed her as she slept.
Cops have pieced together exactly how last Tuesday's massacre unfolded using pathologists' reports and footage from five CCTV cameras security-obsessed Foster had had installed in the home.
A source said: "He'd been drinking all day at a clay shoot, then carried on when he got back home.
"He wanted to kill everything in sight and destroy everything he owned. His wife and daughter were simply executed."
Kirstie was facing her PC when she was shot dead at around 1am. Shortly after wife Jill, 49, was killed - again, with a single wound to the head.
Foster also killed the family dog - asleep on the bedroom floor - before heading out to destroy the property.
CCTV footage captures him driving to the mansion gates with a horse box where he shot out two tyres, to hamper firemen and rescue crews.
He shot three more dogs then dragged their bodies into the main house. Cops have established that crazed Foster had run a length of pipe from an oil tank into the house.
As he pumped flammable accelerant into the house, he torched the house annexe before shooting beloved horses Bramble, Breezy and Scrumpy Jack.
Footage shows him leaving the blazing stables and running back toward the main house, still carrying the .22 rifle.
Cops believe he then barricaded himself into the house with furniture before running up to his bedroom, pouring more flammable liquid as he went.
In the room and next to his dead wife, Foster then lit a match before turning the gun on himself, cops believe.
The family's incinerated bodies were identified by dental records.