I would depend on how benign the crime was. I have relatives live "off the grid". They have weapon arsenols on thier basements, don't pay taxes, fish and hunt off-season, and grow pot to subsidize thier frontier lifestyle.
And even for not-so-benign crimes: I have a girlfriend whose mother was murdered by her boyfriend right in front of her as a child. Due to sloppy police work, native prejudice, and no gun recovery, dude was let go. My friend told her uncle, and her uncle made dude disappear. Permanently. I am not an advocate of taking the law into your own hands, I am not for the death penalty, but if I had names, dates, and descriptions I would certainly keep that to myself and not report it. To see the anguish my friend has gone through to this day; the drugs, the alcohol, the nightmares, the poor decision-making, I'm not exactly sad that that puny little man doesn't exist anymore.
So, to answer the original question: let's just say I have just enough of a vigilante bone and enough distaste for law enforcement in general to not.