This case immediately reminded me of one I saw on Forensic Files!
On Halloween night in 2004, Adriane Insogna and Leslie Mazzara were brutally murdered in their Napa, California home. There were three young women sharing the home, and the two roommates in upstairs bedrooms were stabbed to death. The roommate sleeping downstairs survived. After looking at all the usual suspects (ex-boyfriends, a handyman who was in the house earlier that day, etc.) -- investigators were able to lift DNA from cigarettes that they found outside the home. Shockingly, the killer was found to be the boyfriend to one of the victim's friends. The motive was jealousy.
I have the feeling the ending to this case will shock us too.
Nightmare in Napa
Forensic Files, Season 12 Episode 10