There are times when police officers must use force. Someone they're arresting may be fighting with them or have a weapon that they won't drop. Officers may have no choice but to use their K9 to capture someone who is fleeing, or they may have to pin someone down with their bodyweight to arrest them because of the individual's aggression toward them.
In many situations, it's true that the police have to use force. That force may be justified. However, there is also a point where some people go too far. Injuring a compliant individual, using force to intentionally injure someone who was not a threat and other situations could amount to police brutality.