There was an episode of forensic files where someone was smothered and they were able to match the grease pattern on the pillow to a face that had been smothered. It was crazy. There are definitely ways to figure that out by forensics.
Personally, I think she got fed up and confronted him about cheating. I’m sure she knew. Most women aren’t dumb to this stuff, just many put up with it until they can figure out how to leave or continue to live in denial.
When I was attacked it was because I had confronted my boyfriend over cheating and messaged both girls to tell them. He went from totally normal, at 6 am, to the scariest thing I have ever seen in about 30 seconds.
When psycho manipulators feel their world crumbling around them, losing control, they in turn lose control of themselves completely.