Who Is At Fault For Their Dog’s Death?-OPP

A couple was on vacation and their dog was killed while they were gone. Based on the circumstances of how their dog died, they feel the dog sitter is at fault.

Mike & Kate- My wife and I are devastated because our English Bulldog was killed while we're on the vacation last week. We were in Hawaii for the week and got a call last Friday night that our dog had been killed by a pack of raccoons.
We left our dog, Lola, with a my co-worker. She also has two dogs and offered to watch ours. Normally we had my brother come to our house and stay with Lola when we were gone. But he was hunting last week and my co worker offered. As we were at the airport in Honolulu last Friday getting ready to come home we got the call about Lola.
I know this sounds like a freak accident but my wife and I are livid with my co-worker. We gave her strict instructions not to leave Lola outside by herself. Being a strange yard we were worried she would try and get out. We also never let her out in the yard by herself. We explained this to my co-worker beforehand so she could say no. She said that was fine because she is the same way with her dogs. They're never out alone.
Well when we got home Saturday we went over to her house and got the entire story. Apparently she left Lola outside for over an hour because the three dogs were fighting so she put ours outside. An hour later she heard noises coming from outside and came out to find Lola being attacked by 4-5 raccoon. One of them had her by the neck and severed her artery. She was gone before they could even get her to the ER vet.
We had to break our 3 kids' hearts when we told them about Lola. And we feel all this could have been avoided if she had not put Lola outside by herself. She agreed to our terms ahead of time. It's been awkward at work. Some of my co-workers think we're being ridiculous. It was a freak accident. Others agree with us. This was avoidable and my co-worker is responsible. I know we can't get our dog back but we want her to take responsibility. She is acting like, "Oh-well."  All she said was sorry as we were standing there bawling our eyes out.
Are we justified in being upset with her?

Thanks,
Mark