The main counterargument is how would that help? The violent cop would still be violent. If the police were abolished they'd sign up as a violent gang member, and if a new not-police body is created they'd sign up to that instead.
There is always a most-violent gang on the streets; we just call them the police if they are state-sponsored and ask them to follow the law rather than their own opinions. Defunding the police won't help that dynamic.