As for the OP, it would have to be a VERY severe event to myself or someone I loved, and I would have to deem the threat to be continuous.  If someone's in jail for life or otherwise not a threat, I'd let them rot.  A drunk driver I'd probably let go with whatever punishment the court laid on him/her.
But say I had a daughter, she was forcefully raped, and the rapist got off on some bullshit technicality and was all snooty, like he was just gearing up for his next victim.  I don't know what I'd do in that situation, but it would have to be something that severe or worse for me to even think about it.
And if I did it I wouldn't be making a martyr of myself.  I would go batman on their ass.  I'd map out my victim's life, find out when they're most vulnerable.  Every item to be used in the operation would be bought with cash in different states, stored off my property, accessed off my property, used once and only once, and then, whether the attempt succeeded or failed (in the event of failure buy totally new equipment), would be totally destroyed and the remains dispersed across multiple states when the operation was complete (one of the few times I might make use of a gun buy-back where they melt down the guns).
It might take  a year or two to put together, but it would be cold, professional, I'd have put the Navy SEALs to shame when it was done and no one would be the wiser.