Bottom line - do you believe the husband did it?
https://upload.wikimedia.org/wikipedia/en/d/dd/The_Staircase.png
Bearing in mind he is the only person alive who 100% knows the truth and that the evidence is circumstantial, short of daming, it boils down to whether you believe the husband's account of events or not. Personally I don't and I think he caused her death, possibly deciding not to save it and let her bleed to death.
Everything about him: his attitude and demeanour, what he says, how he says it, his body language, his decisions - reeks of guilt. In short, my gut, says he is lying and that I can see through the smokescreen he has been putting out. Compare that with the man in the film, Greg Taylor, who was found innocent after 17 years in jail - a sincere, credible guy.
Do you believe the husband did it?