Please don't feel guilty about what seems to me to be quite normal thoughts, given the circumstances (as if anything is "normal" in your circumstances).
When I was facing a potential life ending diagnosis, lung cancer, I had a lot of thoughts that would be, to say the least, unpalatable to my loved ones.
Things like if I died in service (as a civil servant) my named beneficiary would receive three times my annual salary, so better to stop my moves to actuarily resign, like five years left would be ok(ish), like could I stockpile the pills I was sure I'd get (I did) to ensure a painless death.
I'm one of the very very lucky ones. I survived.
But I know how "dark" my thoughts got. Yet I didnt, and dont, see them as dark. I see them as a rational response to a bloody awful situation.
And I see your thoughts in the same way.
I'm so sorry you and your loved one is facing this. But neither of you have a choice in the matter, you must both face it. A very unmumsnet hug.