We are all different and of course we are looking at this from the outside and with the benefit of hindsight.
If I was Bamber, I would have been thinking to put it blunt.
" Fuck sake I shot her twice "
" Fuck sake, I went to town on dad, I went overboard with him more than I should, will they believe Shelia did this?
By that time though he had done what he had done. He also got a favourable result with the pathologist who said that two shot suicides are not common but not unheard of.