Last time I saw a shoplifter, he was running out the store (a K-Mart, as I recall) with two people chasing him. I chased him down in the parking lot, headed him off, and ran along while convincing him to just stop, as there was no way he was going to get away from me. He finally stopped and we just stood there for a while. He was a young kid. Maybe 15. Out of breath, scared to death. Security guard ran up and hit him. I grabbed the security guard and told him to knock it off, the kid gave up. Just stop. The other guy came up and they took the kid back into the store. Don't know what happened to him. I went back to the car and my wife told me I was nuts. My kids asked me what I was doing. I told them I was helping someone with their car or something.
But standing there and doing or saying nothing just isn't possible for me.
Couple years ago, we saw a hit and run right in front of us. I took off after the running car, chased it down, convinced them to pull over, took a picture of their license plate and the driver and they took off again. I let it go, as I didn't want to be driving like that putting others at risk. Went back to the accident scene and waited for the cops to arrive. Turns out a passenger in the hit car was an old coworker. Gave the cops the info.
Again, probably not a good idea. It'll probably come back to bite me someday, but standing by and not doing or saying something is part of the problem.