Oluwaseyi Dada, 21, and 17-year old Katrina Makunova were dating but their relationship had deteriorated and is described as having a "background of animosity".
A series of social media communications between the couple obtained by the police from Katrina's phone showed that they had "made threats and insulted each other via social media".
On Thursday, 12 July 2018, Katrina had gone to the lobby of a block of flats in Camberwell, south east London where Dada lived. They encountered each other on the ground floor and a struggle began. During the struggle Katrina fell onto the handbag, causing a knife she put in the bag to pierce the bag and stab her through the chest.
Police and paramedics were called to the scene where they found Dada performing CPR on the victim. Despite the efforts of paramedics, she was pronounced dead at the scene at 13:02hrs.
Dada was arrested on suspicion of murder at the scene and subsequently charged with manslaughter on Friday, 13 July.
Dada was due to stand trial at Southwark Crown Court for manslaughter but he admitted the offence yesterday, January 10, 2019.
Dada told cops he feared Katrina was about to attack him and she fell to the floor when he grabbed her handbag. He then told them the victim fell onto the six-inch blade in the bag and it pierced her.
Dada, from Streatham, admitted manslaughter and will return for sentence on January 29.