Assaulting a thief whose committing a crime?

Yes on both. You are the one committing the crimes. When you say attempting a citizens arrest and you have no right to arrest someone for checking car doors. With an actual crime, it's likely that the prosecutor would not file charges, but the criminal could sue you and may win. Then you could pay 1000s of $. You would have to pay the hospital bills and basically like when a cop calls for a ambulance, except it would be on your insurance, since your a independent police and jail dept. and all.