We've had a case here in Northern England where a rapist was followed by two teenagers, who had heard the screams and cornered him. The trial happened to be on the same day as the first website went up in this Country naming and showing five paedophiles, but relevantly telling you not to approach them.
I could see the difficulty the police spokesman had in both commending the boys' action, whilst sticking to the "don't approach" line.
I think that you did right, in this age you cannot take the risk of tackling/getting involved. Years ago, when the likelyhood of firearms was much less, I rugger tackled a shoplifter coming out of a DIY store, but:
a) I wouldn't do it now.
b) I knew that I had four burly friends standing by a nearby car.
c) It was worth it, both because the store Manager bought us a round in our pub nearby and I got some store vouchers.

The one thing that I don't think has surfaced on this thread was, that my main concern was that he would recognise me in the street, perhaps when I was on my own and he was with a bunch of thugs.