...which in turn undermines the theme of morality. Vash's character was that he'd go out of his way in every extreme to uphold his character... killing that one fellow (Bluesummers? something like that) was very odd. One could argue Vash's killing of Bluesummers was the extreme - however, Vash's reasoning throughout the series is that each party can survive mutually without harm to the other (re: butterfly and the spider). I understand there's a great deal of symbolic items in that, however, as I delineate further down, the construction of the actual series detracts from what message it is trying to convey.
What's really strange (as commented before by others) is that he dwells on this for only one episode, and not all of it either. Also very out of character...
I stand behind my first comment - it was good, but not great, and I'll add to it - I don't think it should be mentioned in the same breath as other 'better' anime (re: greatest ever comment). It's really disjointed, as though the writers decided half-way through they need to make it serious, otherwise it wouldn't be good anime. Knives shows up out of the blue, as does the serious theme. It would have been better served if they toned down the comedy OR introduced some more serious elements in the first half of the series (ala Cowboy Bebop) - I remember the first time through being very unprepared for the drastic change in mood.