But surely if you think that it is in some cases necessary you should be willing to do it yourself, rather than simply expecting others to do it for you? Even if, in practice, there's no reason you'd need to do it.
Nope, doing it this way will create chaos and many people will come out wanting to take their justice them selves. If there is a law, one should refer to the law and see what it has to offer, and this is mentioned in the Quran. Applying the law should be done in a regulated and measured manner by those in charge. It would be done for the community, not me as a single person. If it is done for me, in a case concerning my right as an individual, I would first consider forgiving the culprit if I see they desert it instead of letting them executed. May death sentences are actually dropped like this in Islam. If I can't forgive, then I should let the authorities take care of it.
The only way I would (or willing to, as you asked) do anything to an offender is if I had no choice, like in protecting myself, my family or those instantly needing my help, and even then, it would be with priorities in reaching safety, not killing.