Why are Rocco and Il Duce not considered 'scum'?

God it's been ages since I've seen this, but IIRC, Rocco was only a delivery guy. He never really got his hands dirty, and only ever was given 1 task to "make" him. As such, he was also seen as someone who had information. The brothers most likely kept him alive for these reasons.

As for Il Duce, it may be because they realized he was their father. The scene where they are praying over Rocco, Duce walks in, hears what they are saying, and then finishes the prayer for them. He then joins them in their cause.

Il duce wouldn't kill women or children, and when he is introduced into the plot it is said that "he had a thing for killing wiseguys". You could say that he was a hitman with a moral code, which just happens to align quite well with the MacManus brothers own sense of morality. The brothers didn't know this at their first encounter (nor did Il Duce) but when they realize they are family with the same goal in life I guess it makes sense to join forces.

As for Rocco. A bit harder to explain, but Rocco isn't evil, he just works for evil men. IIRC in the scene in the kitchen where they start talking about doing this for real, they say something along the lines of them deciding who's evil and not (and thus, gets to live or not).