I'm sorry for your loss. That can be a real shock.
The way I see it, there are two ways to keep chickens (with some variations on each). One is free range, and know that you risk some losses at any time. The other is to build a secure run and keep the chickens in all the time. Neither is right or wrong, just individual tolerance levels. You do need to understand, however, that if you free range, you are taking risks. Yes, you can shoot or trap any animal that takes some of your birds. Usually, though that happens after you have had birds killed, just as in your case. Then you get rid of one, but there will be more. If you aren't willing to accept that risk, build a large secure run. It is possible to have healthy, happy chickens in a run, if they have enough room and enrichment.