You don't need a reason to divorce him, but it sounds like you've probably got loads due to him even asking about leaving you.
He's emotionally and mentally dim if he can't understand that you needed observing after the operation. To even ask to relieve himself of guilt just shows what a selfish, entitled shit he must have been for many years.
He's a fool, like many on here who think it's clever to respond with the "you should have said something then', darvo at it's finest.
If you are too thick to understand someone in her 50's ( aneasthetics can be particularly dangerous for certain age groups, ie, teenagers and menopausal women) needs and should expect care by her primary partner, then you shouldn't be in a relationship. What's the point of you.
Marriage shouldn't be like this, awards given for how much someone can dish out neglect or medals for who can ignor their partner is a cunt the most.
It's thoughtless, uncaring and irresponsible.
Op if you stay together, promise yourself that if he ever goes into surgery, you will not be the one who takes care of him, whislt in the hospital and on his return. Make sure the dog is left with him and preferably contact no one so no one else has the oportunity of caring for him.
And make sure the day of the operation, you go on a day trip, return home and leave him in the same circumstances, promise to phone him to ask how he is and then don't.
Got it.
Op will probably not do this, but she should because he did, he a callous twat.