I think saying Debbie wasn't actually married to Cameron is nitpicking and so is saying that Pete didn't set out to murder Ross or even that Ross isn't dead because Pete laid into Ross in anger and the result was that he is dead as far as Pete is concerned.
The crucial point for me is that on believing he had killed his brother Pete then hid the body in the boot of his car and disposed of it in a wood. All classic Cameron type stuff if you ask me, particularly that he is now prepared to lie to his wife, his family and the police.
I think telling Debbie he never wants to hear Ross's name demonstrates how despicable he has become. Though he cries a lot and acts more emotional than Cameron he is still turning out to be like him.
As for Debbie I hate that she constantly brings this level of drama and dreadfulness on herself and her family and yet always gets off Scot free herself. Why is Pete more or less forgiving her

and yet beats Ross to his death

This was a perfect opportunity to get rid of Debbie but instead we're stuck her with and stuck with her going through the same boring
married to a killer rigmarole