It's the hard end of chicken keeping RC.

It's there for everyone eventually. It doesn't matter how hard one tries, chickens get sick and injured and predated.
Feeling guilty really isn't a forward step and it doesn't help you, or the chickens.
Most of us do the best we can. What can be difficult is to find out our best isn't good enough. It's a worthwhile lesson to learn. I know many people who still haven't learn't this and when pressed will tell you they weren't really trying etc etc. Very few people I've met have set on a task with the intention of failing at it.
If I could have done, I would have done, but I didn't so I couldn't.