Of course it is natural to be angry and to look for blame when a loved one dies. Researchers have determined that it is a large part of the grieving process.
It is an unfortunate fact of life that a hip break/fracture in an elderly person can hasten death. This is particularly true of female patients and those with degenerative health conditions. It is thought that the pain, surgery and recovery simply depletes a patients resources, the remaining energy they have which is keeping them alive.
I spent six years working in nursing homes as a student and it was so common that symptoms of a UTI went unreported by the resident. Women born in times past simply did not talk about things 'down there'. Not like we do here on MN. It would often take a member of staff noticing that the contents of the commode were a little dark/strong for an infection to be determined. Also, bear in mind that many residents were unable to communicate meaningfully so could not inform us of any discomfort.
When someone is 'pre-death' (preparing to die) or 'actively dying', they will only be able to manage a sip or two, a couple of nibbles. They will be tired and weak, sleeping often.
As awful as this sounds, what happened to your gran was simply 'facts of life'. There's no way I can dress it up, this is how a natural death occurs. We might be used to there being a concrete cause of dying, cancer or a road accident. When there is no one factor to blame, we can and do dwell on the 'what ifs'. I do not mean to be crass her but perhaps consider the 'what ifs' of your gran surviving. Dying as she did was far more dignified and merciful than a death in the late stages of dementia.
As for visiting her, you mustn't feel that you have to. A body is just a set of overalls really. Not the beautiful soul she clearly was. Not the memories you have and not the love you shared.