After the death of a loved one, feelings can often be ambivalent. That is grief. There may also be some sense of relief. Whether the relief comes from the feelings that the loved one is no longer suffering, from not having to witness that suffering, or from not having to care for the person any longer, such reactions are human and natural.
However, most people don't see it that way. They feel guilty about these natural feelings. Could they have really loved the person if one part of them feel relieved by the death? They may blame themselves for not having done more, for having uttered some cross words, for not having been at the person's side more.
Self-blame is a no-win deal. No matter what miracles of caring or nurturing the bereaved may have performed for the deceased, there will ALWAYS be gaps in the record and hence room for self-doubts. The more a survivor broods on what might have been, the worse things get psychologically.