We lost a hamster to one of our cats a year ago. Her door must have been left open and she decided to go exploring. She was such a great little gal, even played ball. We miss her a lot.
We were very mad at ourselves, and the cat, but ultimately its a fact of life that owning prey and predator is risky. She died quickly and we are thankful for that.
Your heart will heal with time and the guilt does subside. We've not made the same mistake since.