I would not think the male killed her. I am not sure they will or could break another adults neck.
In my early years of breeding odatria, I often mentioned all manner of accidents. Monitors are curious and often get caught in odd situations and off themselves. Once dead, the others chew them up.
Taking a male out, could cause a female to seek him out and try to find ways to get to him. (seperation anxiety)
The understanding of pairs with reptiles is totally odd. People are the goofy ones. With reptiles, pairs stay around eachother, around means, within 5 to 30 meters, depending on size of the species. To understand it, at times they are within close proximity 5 to 30 meters, at other times they attend eachother. Consider a monitor can move 30 meters in a few minutes or less. I imagine they can smell eachother at that distance.
The above causes lots of problems for keepers, they are under the impression that to be a pair, they must be glued at the hip and stay within inches for their entire lifes.
In your case, she may have had a need to be in attendance with her chosen male. This caused her to do something not normal. Or caused her to simply expire. In this case, I am sure she was already compromised and the added stress was more then she could handle. Good luck