Police recorded the footage after Sean Martin, 21, was found stabbed to death in the hallway of his girl-friend's 17th floor flat in a tower block in Coatbridge, Lanarkshire.
Shaniece Dobson, 21, of Motherwell, denies murdering Sean by striking him on the body with a pen and a knife on February 25 last year.
The accused sobbed in the dock as the gory images were screened at the High Court in Livingston.
Sean's sister Ann, 17, said the atmosphere in the flat changed after the couple returned with a takeaway meal.
She believed the couple were arguing over their ex-partners and over a can of cider.
She said Dobson suddenly grabbed a pen and tried to stab her brother.
She told the jury: "Sean said: 'What are you doing, you cow, trying to stab me?' He pushed Shaniece and she pushed him back and then Shaniece went into the kitchen and grabbed something next to the window.
"She came back with a knife. She went up as if to give him a cuddle then she did that with the knife."
Miss Martin made a stabbing motion with her fist clenched as if holding a knife pointing downwards.
The trial before Lord Boyd continues.