The wound looks like a mix of stabs and cuts. The amount of blood on her with no other visible injuries most likely meant she had to bleed out that hole slowly. He obviously didn't have enough follow through to make the death process quicker otherwise he would have continued until decapitation, I imagine all his "sorry Bianca's" means he probably had regret either during the attack or immediately following (I would imagine the former because he didn't make the process quicker and I don't imagine he did this act with the hope of prolonged torture otherwise he would most likely not express remorse directly following the incident) . Id like to know how she doesn't have any other defence wounds visible on arms or body (If there are any other pics it could help shed some light, preferably of her hands post mortem) I'd imagine she was either bound or he got close to her with the knife through the guise of roleplay (because it would appear she at least elluded to having rape fantasy/ knife fetish) I think we can rule out the whole "in the moment/crime of passion" because I really don't see an excessive amount of brutality in the wounds. The size of that blade vs the wound he could have made those in like two or three stabs. So if I had to guess he was planning some big elaborate death for her but once the attack started he got cold feet and pussd out like a bitch (which would suck extra for her, the initial shock probably wore off and the pain started setting in while he sobbed like a baby)
Also I have to throw in a tad bit of levity here otherwise I won't be able to sleep tonight (sorry null)