Deceased, intentional, wondering if he might be a serial killer.
I do wonder if he has done this before or something similar but did not go quite as far.
I don't believe this was exactly a crime of passion; I don't think it was entirely impulsive.
I believe he intended to violate Danielle in some way that day, and when he realized she wasn't going to go along with it and thus he was not going to get his way, he decided she had to be punished. Like, " If you don't do as I say, you will regret it." I do suspect he got angry, but I don't think he completely lost it. And I believe once he made a decision on how he was going to punish her -I'm sorry, but no, I do not believe she is with us any longer ...-, he rather enjoyed it.
I don't believe this was one of those situations in which say, a man tries to rape a woman, she refuses, he freaks out and kills her out of fear that she will go to LE, he will likely be arrested, his wife and everyone else is going to find out, etc. I think this was more about -in his twisted mind- her not respecting him.
I do believe he had help, and whatever role this person played (transporting her, giving him a ride at some point, etc.), they didn't know what they were helping him with. I do think that by now, the person would have put the two and two together but they may be too afraid of the perpetrator to come forward.
Purely speculative on my part.
ETA: And I do think that he was rather meticulous in covering it up, but not enough. They will get him.