Nobody can give you "advice" on this without taking on some legal responsibility.
In most states.... it is actually illegal to give legal advice and this would probably be covered..
However..... Being armed and coming into contact with someone that needs to be stopped is a tricky situation.
You better be sure you know what is going on and that a law was broken. If someone claims a crime happened and you jack someone up and hold them at gun point..... YOU could be charged with Abduction, Brandishing, and other charges.
Most off duty cops are "good witnesses" and do not get personallyinvolved. About the only time they do is if they need to stop someone from getting hurt or killed. This can clearly be identified by most anyone... citizens included.