I admire your deontological zealotry. That said, I think there is an implied virtuous aspect of "internet vigilantism" that feels ignored (i.e. disabling a malicious bot means it does not visit other sites) While I do not absolve anyone from taking full responsibility for their actions, I have a suspicion that terrorists do a bit more than just avert a greater wrong--otherwise, please sign me up!
"Hurting people is wrong, so you should not defend yourself when attacked."
"Imprisoning people is wrong, so we should not imprison thieves."
Also the modern telling of Robin Hood seems to be pretty generally celebrated.
Two wrongs may not make a right, but often enough a smaller wrong is the best recourse we have to avert a greater wrong.
The spirit of the proverb is referring to wrongs which are unrelated to one another, especially when using one to excuse another.