In terms of accountability I cannot think of any country where the police force is actually accountable for anything they do. Abuse of power pretty much everywhere and the police gets a blank check.
True. But most countries give their police more powers to abuse and somehow they see less of their police power abuse involving dead people. It can't be a direct reason (police certainly don't shoot to kill because they need to fulfill some imagined power abuse quota), but I suspect that the shootings are indirectly rooted in Americans expecting their police to do their jobs without ever doing more that polite chitchat unless they declare someone a clear and imminent danger. I'd rather be harassed than shot at.
I think the book "the rise of the warrior cop" explains that how the police does their job in the US has changed and why. It gave pretty satisfying answers as to why it's become what it is now.