Of course, if the next bystander sees the cops running after two people and body-slams the wrong one, he’s opening up a can of worms. Which goes back to my comment - unless you know what’s really going on, don’t jump in.
It depends what the crime and outcome are. Force should be proportionate to threat. If the fellow was shoplifting a candy bar and the body slam leaves him quadriplegic, there’s a chance the jury may not be sympathetic to the Good Samaritan.
I have a friend who’s house was burglarized. He called the sheriff’s department and they sent out a deputy to take the report. After the deputy left and my friend was headed back to work, he noticed the car of his house cleaner parked behind a nearby church. Further down the road he saw another car pulled off on the side of the road and the housekeeper was getting into the backseat. My friend in a large pick-up truck, ran this car off the road and held 3 people at gunpoint, while he called the sheriff’s office again. The man driving the car was irate. When the deputy arrived at the scene, it was determined that couple in the car didn’t know the housekeeper, but had stopped to give her a ride. The car owner wanted to press charges against my friend, but the deputy advised him that it was unlikely the prosecutor would take the case as my friend had been robbed and was attempting to keep the burglar from fleeing. No charges were brought against my friend, but his housekeeper was arrested for burglary, she had jewelry and cash on her that she had stolen.
Laws vary by state regarding the power of citizen’s arrest. In my state a citizen may arrest for any felony or a breach of the peace committed in his presence. Once that happens, the citizen has the same power to use reasonable force to effectuate that arrest as a police officer does.
So long as the citizen keeps his use of force reasonable, then he is not guilty of an assault or battery or any tort as his use of force was privileged.
But I agree with others that this is generally a very bad idea as in that split second it is hard to determine who is the good guy and who is the bad guy.