Can animals attack or kill people even if unprovoked?

[D]

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For example my friend Kerry has a rottweiler that is as soft as shit, before going to her house for a dinner party for the first time she told me "Don't touch Stella's belly she doesn't like it and she might bite".

If Kerry hadn't told me that and I had touched the dog's belly and she bit me who is at fault?, me or the dog?

Kerry's fault 100%.
Knowing full and well that her dog could bite anyone if they touched her belly and did not put the dog away is irresponsible. Telling someone or not has no bearing. She should take precautionary measures regardless of how sweet the dog is EXCEPT if you pet it's stomach.

People cannot be expected to remember your rules for petting the animals at all times. People forget and go to their own habits too easily.

I actually disagree with you here. If someone tells me not to pet their dog, or not to touch a certain spot on their dog, I don't. Habits or not. Why? Because each dog is an individual, and I treat them thus.