Well, for starters, European police are actually trained professionals (in general, much more so than American police) and have different oversight. American police also handle a wider variety of things that really aren't law enforcement - things that should be handled by other kinds of professionals.

EDIT: American law enforcement agencies are also home to some of the highest rates of domestic violence perpetrators and right-wing extremism.

American police shoot and kill 3-4 people each day. That doesn't take into account deaths that occur in jails and prisons due to negligence.