On the original topic:
In UA - I agree with the OP that it was morally wrong for Pepe to do the lemon trick on Andy, but OTOH it was in character for Pepe to do so, since he is a street fighter and always will be. I don't see it as 'just desserts' but as the streets of AM still being not a completely lawful place (despite the efforts of the ever expanding Watch).
But I disagree about Snuff. Stratford escaped from custody of the Law en route to trial. By doing so he morally gave up whatever protections the Law offers to alleged criminals. A lone civilian, even one as skilled and capable as Willikins, is not required to perform a citizen's arrest under these conditions, and the level of technology of the AM environs does not allow him to call for a sufficient force of the Watch to perform an arrest in time to stop Stratford before he commits yet another murder. Also, the whole thing happens in open country, away from a Watch House. This is more similar to the situation 71-Hour-Ahmed described in Jingo - his beat was too large to police entirely by methods that protect the rights of the suspects. Unless anyone is suggesting Willikins engineered Stratford's escape so as to be able to kill him, I don't think he did wrong for that particular situation. In a city it would have been different - if he just saw Stratford he should have called the Watch. Then again, if he saw Stratford at the Vimes home he would have been justified in killing him outright.
YES YES! And let's not forget that it was Stratford who attacked Willikins, not the other way around. It was self-defence, not vigilantism, any way you look at it. Besides, Willikins did not set out specifically to kill Stratford, he just hovered nearby to intervene in case something went wrong. All Stratford had to to in order to escape the wrath of the Groom of Doom was to refrain from escaping and killing his guards, or, at the very least, refrain from trying to kill Willikins. Can't say fairer than that.
Yes, it would have been better for Stratford to stand trial, but since he did escape Willikins was justified in killing him.