Well first of all, the guy never opened up about his past like that hence why he was living in the home with her. They had been dating for a while and like most people they don't pull out background checks, they learn to trust their significant other. Secondly, he never showed any signs of violence prior to that to the Vic or her children (one not listed). In fact was complete opposite toward her, treated her very well actually. Furthermore, she was in shock after being stabbed and did not remember any of the  events that occurred that night until the following day so of course she would tell the cop that in agreeing with her boyfriend. That old saying there are 2 sides to every story, before you run off and have something negative to say about a young lady learn the entirety of the story first. 
-The older sister of the Vic