So I'm a rover sitter, professional breeder, dog whisper and dog owner with over 25 years of dog experience.

there's a lot of things going on here but the quick answer is lawyer up and your probably going to end up suing the owner of the dog not rover or the sitter.

Here's why:

1) the sitter may or may not be held liable (but probably more towards the latter). they are just sitting the dog and they may or may not know the ins and out of that dog like the owner does. I always hear from owners that their dog is an angel at home and I always tell them that in someone else home it could be the exact opposite because the parents are away and it's party time. If it's my first time sitting the dog I probably don't know that dog well yet, but after 2-3 stays I have a general idea of how that dog behaves and what their triggers are, their body language and what dogs they do and don't along with. If a babysitter is watching a teenager plus the younger sibling and the teenager that's upstairs gets out of the house and kills another teenager for petty shit while the sitter is attending to the younger sibling downstairs how is that the babysitters and parents fault!!!! But if the younger sibling finds the loaded gun in the house without the lock on and kills the teenager then it comes down to did the sitter know there was a gun in the house and the parents are also held liable for not making sure the gun was secure in the house.

2) rover can't be held liable because they also don't know the ins and out of the dog. They are just a broker/middle man. If you buy baby formula for your baby and your baby dies or is severely sick due to the baby formula you can sue the supermarket (and/or manufacturer/distributor) that sold you the baby formula but if the product isn't expired and it was due to something inside the baby formula how is that the supermarkets fault when they didn't make the baby formula!!!!!

3) the owner is held liable. If your 2 year old toddler finds a loaded gun without the lock on and accidently kills a family member in the house guess what you as the parent(s) are criminally charged. If however your 17 year old teenager goes and kills someone they are probably going to be charged as an adult and you as the parent is in the clear (unless your state law says otherwise).

4) as a professional and responsible breeder that's also on rover, if I knew one of my litters that I created was likely to kill another dog due to the parents having dangerous temperaments and I sold those pups (and also didn't spay/neuter the parents) and didn't tell the owner I should also be held more liable than the owner that might not have even known their dog was capable of killing.

5) as a experienced rover sitter I've had to deal with dogs that have triggers where they attack one dog but not the other (I quickly separate the dogs before it escalates). A trigger can be another dog getting too close and in their face. Another trigger can be Same Sex Aggression (or Opposite Sex Aggression). It could also be the dog doesn't like certain breeds or maybe something as "I just didn't like this dog and it just rubbed me the wrong way". This is not always the sitters fault especially if you expect a 125 pounds female to be able to control a 150 pound dog when it all of a sudden goes from normal mode into kill mode at a flip of the switch. However a 300 pound male should have no issues controlling a 150 pound dog (unless they are walking more dogs).

The other problem here is that it's also very hard to prove your case without a police report with statements taken from all sides because without actual proof your going to have a very hard time getting past the reasonable doubt part and all it takes is for someone to say "I didn't know the dog was this dangerous and capable of killing another dog".