Vanlauredel
That was fast. Was it Danny? It was probably was I could see it. Why he did it. I mean not everyone can be like Sam maintaining a purely physical relationship. As Levi said Danny is a normal guy, of course he'd fall in love with Sam. Which normal guy wouldn't? She is beautiful and intelligent. He probably thought they were in a relationship but Sam didn't want to make it "Official" or "put a label on it" as they say. But after the 15th Murder case, when the whole internet saw pictures of Sam and Levi together and started shipping them. And how everyone thought they were in a relationship. Besides Levi is a handsome dude, he was probably felt jealous. He felt betrayed and was angry. But not angry enough to kill her but to scare her. So he wanted to kill someone he scare her, thus Rona the Dog walker and Mo. Hence when he entered her apartment security weren't alerted. Either he had a key, or entered together with the dog walker. He was in the process killing them when Sam came home sooner then anticipated. Since he was caught in the act he panicked and decided to kill her too. Because the security knew about him they didn't act when he entered the house, besides Sam came home earlier tham usual so they probably thought she called him there. Makes sense now.