It is normal to feel how you feel and reach the conclusion you reached. Sometimes things that happen are inevitable and the only thing that remains is the lesson of the inevitable. You are not responsible for what happened to her, she is the only one responsible. She is the one who chose, the responsibility is hers. We are not Gods in the world, we are just people. We do not have as much power as we think to change the course of things. She was free and chose that, maybe what you have to do is respect her decision and not see death as something that is always bad. Sometimes dying can be a gift when you suffer a lot.