Can i justifiably assault a cop if he's killing someone in custody or committing police brutality and not get charged because I was stopping a crime.

Interesting, and scary, question. the law usually requires similar force when defending yourself, so if someone is attacking you with their hands, you can fight back with your hands. But oif you pull a gun out and shoot them, that may be a problem.  In your scenario, you want to stop a crime (i.e. attempted murder) by a cop.  I understand your feelings, but I believ you would hav eto call the police and make an emergency report.  If you attack the officer or inetrfere in some physical way, you may be in alot worse trouble.  Your best option is to call 911, get people nearby to witness the event, have everyone demand he stops the assault and video it.   This is not legal advice nor does it create an attorney client relationship. It is just informational. You should hire an attorney if you want to pursue a legal matter.