Lack of plausible evidence, I guess.
These days, attempting to kiss a young lady or place a hand behind her back, is probably not what most people would regard as "sexual" in the context of criminal behaviour.
If there was compelling evidence that he had removed her clothing, taken photos of her naked, performed some sort of sexual act with (or over) her, then I would agree that the motive was sexual.
But in the absence of any of that, I can see no more reason why he would want to kill JY than I can see any reason why she would want to scream incessantly if he had made a pass at her.
Unless there is much more that we don't know about, I'm afraid that I am coming to the conclusion that this case is about the interaction of two hyper-nervous people:
VT, apparently very inexperienced with women
JY, apparently half paranoid at the thought of being alone at the age of 25
The two lived either side of the same wall. They were neighbours who had never even bothered to introduce themselves to each other or to invite each other round for a drink and a chat.
My biggest fear is that this is how some (particularly young) people live today. Double lock the doors. Avoid being part of the community in which they have chosen to live. Would JY be alive today, and VT free of jail, if the two couples hadn't simply done what people of my generation do: welcome each other as neighbours, get to know each other, help each other out if the cat needs looking after ... it is now too late to know.
Demanding or forcing a kiss and putting an arm around someone who doesn't want any such contact is sexual and grounds for complaint of a sexual assault.
Joanna may have had a fear of being alone for the weekend in that flat. I don't know why mention that she is 25. It is a sort of fear that many women of all ages would recognise. As it turns out, if she was afraid, she had good reason to be.
As far as I know, Joanna and Greg lived good decent responsible lives wherever they chose to live and I don't think any criticism attaches to them for their contribution in society. I think it is unhelpful to say the least to attribute some sort of blame or speculation based on some highly personal view as to how they should live and as to their relations with neighbours. There is nothing to say that Joanna would have been safer had she had more to do with the man who killed her.