"Mad Dog" had a lot to follow on from such an outstanding three parter that had immediately preceeded it. Already high expectations were heightened because although there were a few disappointments there were a lot of great and more Season 7 episodes. Talking about the Season 7s of the original 'Law and Order', 'Special Victims Unit' and 'Criminal Intent', that for this show was easily the best ('Criminal Intent's' was very up and down and while 'Special Victims Unit's' had some fantastic episodes it had some real misfires as well).
While not one of the best episodes of Season 7 or a 'Law and Order' high point, "Mad Dog" is still great and a strong representation of why 'Law and Order' was such a good show in its prime. When reading the synopsis and seeing that McCoy would go to extreme lengths to get a result, part of me did worry as McCoy's unprofessionalism in the first half of Season 5 was a turn off and not done well. This aspect actually was handled remarkably well.
Actually think that "Mad Dog" could have done more with whether Darnell had changed or not or whether something had tipped him over the edge, do agree that that was not addressed enough. It was though handled a lot better than 'Special Victims Unit's' "Demons" that had a similar scenario, at least Darnell was consistently written.
Everything else works wonders. On a visual level, the episode is solid and the intimacy of the photography doesn't get static or too filmed play-like. The music when used is not too over-emphatic and has a melancholic edge that is quite haunting. The direction is sympathetic enough without being leaden, while having enough momentum to make the drama sing in its atmosphere.
The script is tight and has always made me think long and hard after, the moral dilemmas that comes with the lengths McCoy goes to has intensity and intrigue. The story is attention grabbing and never lets go, McCoy does go well over the edge but not infuriatingly so or to the extent that one hates him. This is an example of an episode where the characters wanting to secure a conviction was rootable.
Of the uniformly excellent cast, Sam Waterston has the meatiest material and is both appropriately subtle and ruthless and it is very interesting seeing how McCoy's mind works. Burt Young is unsettlingly sleazy.
Summing up, excellent. 9/10.