That is what I don't understand , that LB had a ongoing relationship with DM and CN was dating the last witness today and they were all friends. I thought the motive for DM wanting to kill Laura was because Laura told CN that they were involved. Things are not making sense at all.
I guess a few months later it was supposed to be over between LB and DM, and CN and DM were supposed to be exclusive. But either it was still going on behind CN's back (my guess), or LB just claimed it was. Making CN jealous, mad, maybe threatening to break up with DM.
My guess is he claimed LB was lying and promised CN to "remove her from our lives". Kind of like a Valentine's present I guess (irony).