IMO the limits lie where someone isn't interested or can't be reformed. If I could turn the Boston Marathon Bombers into adovcates against extremism I would. And if successful they'd likely be very good at it because they'd understand the motives of other extremists better.

It doesn't work with everyone (that's why we have prisons), but AFAIK it's not that uncommon for people to tunr their lives aeound in this way. Most criminals are not inherently sadistic: they do care about other people. Their upbringing and circumstances just lead them to a bad place and they ended up taking that out on someone else.

That doesn't mean they aren't repsonsible for their actions or that they shouldn't be punished, but it doesn mean that they can often be rehabilitated with the right support. And that's generally a win-win scenario where it is possible.