Hearing the noise doesn't mean she heard it clearly. I don't know too many 90+ year olds who have good hearing. It's not like she was 19 with great hearing, eyesight, and quick reflexes. Yes, she had time to pick up the gun but we don't know how accessable she kept it.
If she had a clue that there were druggies coming to the house, or if she was indeed dealing herself, then I could see where she would be skittish about a bunch of men hollering and kicking her door in. If she had reason to think they were druggies, she had reason to fear for her life. I'm giving her the benefit of the doubt.
There are so many different stories floating around. One report was that she saw them approaching. How anyone would know that though........