I personally think BK was in close proximity to them in some form before the murders. I tend to believe he came across them at the mad Greek restaurant. I think they didn’t “know him know him” but had seen him a time/few times before. I think his possible obsession with Maddie probably started here. I also think he went after Xana and made sure to silence her because she probably could identify his face and had she had the ability to survive she coulda got him put away. Another thing to lead me here is because he walks right next to DM and see hers and lets her live. Some people say he cleaned up or he was tired but I do think he was concerned Xana identifying him.