The line is when police mete out punishment, beyond the force needed to capture, restrain and transport the suspects, who have not yet been found and sentenced by a court of law.
In too many instances, they feel an entitlement of meting out some punishment, for the trouble they have been put through to effect the capture.
As I have repeatedly stated, Police need better equipment, tactics and training.
On the surface it looks like a race issue, but that only arises because of the violent criminal behavior of blacks, their frequent contact with LE, etc.