Was Eddard Stark being knightly? If by that you mean adhering to the tenets of law, despite the circumstances, then perhaps. By the way, that black brother didn't serve Lord Stark, he served The Wall- he effectively deserted when his brothers were massacred by white walkers; I can't say I blame him. It is one thing to die in battle fighting a man or men, quite another by supernatural beasts for which no real way to kill was discovered- at least not until Jon Snow thought to use fire in the Lord Commander's solar when one of their black brothers arose as a WW. So, perhaps he was knightly, but I would not say he was just. He automatically assumed the man was mad, and I can't really blame him for that as there was no real precedent...not in hundreds of years, according to canon.
The McManus brothers are noble...I wouldn't say necessarily knightly. They pursued their own brand of justice, and the populace thanked them for it because only hardened criminals and mobsters met their ends at his hands. Although they probably should've killed Rocco too- that man was a loose cannon and as far as I was concerned, just as bad as the mobsters. All people have a redeeming quality if you take the time to learn it, doesn't mean it should absolve them of their crimes.
So, does that make the McManus brothers murderers, or righteous executioners? Who are they, in their supreme arrogance, to assume they know the Will of God? Depends on where you're standing. Still, I loved the movie and I'd support them as well.