Phil, it is totally normal for close relatives to feel guilt when a loved one dies. When my mother died, I had to make an appointment for my oldest daughter to talk to my mother's doctor. He had to reassure her that there was nothing she could have done to save her.
My mother had leukemia. She had been getting chemo, but when they tested, the chemo drugs were about half of what they should have been in her bloodstream. They needed to significantly increase the dosage, but in order to do that, they had to hospitalize her. They did the chemo, and gave her a LOT of anti-side effect drugs. She was doing fine. The next morning, the doctor gave her another medication for the side effects, and mom began acting funny. My daughter witnessed it, and the nurse told her it was not unusual due to the medication she was on. About an hour later, mom was headed to the restroom, and she collapsed. My daughter pushed the call button for a nurse. When mom collapsed, she went into a coma. She never awoke from the coma.
My daughter kept saying that maybe she should have called the doctor, instead of the nurse, when mom was acting so strange. Maybe she should have insisted the nurse call her doctor. She kept thinking that there was something she could have done to change the outcome. It seemed no one could console her, or change her mind. She felt so guilty that she didn't do the right thing to save her grandmother. That's when I called mom's doctor, and made the appointment. We went in to see him, and my daughter got all her questions answered. She also found out there was NOTHING she could have done to change the outcome. He explained to her that there is almost always some type of guilt when a loved one dies, but it was not her burden to carry. She was fine after that.
I have to say this though. Because of mom's reaction to that particular medication, later on when I was getting chemo, my oncologist would not use that one side effect drug on me.