Issue being mainly that the Skyrim Thieves' Guild is literally in the pocket of the most corrupt noble in all of the Rift, Maven Black-Briar. How can they be a Robin Hood -type organization if they actively contribute to corruption and maintaining the status quo of wealth?
On the topic of Sithis, the Dark Brotherhood mistakenly worship another deity, either a Daedric Prince or (it has been argued) Lorkhan. Sithis is a metaphysical force rather than a deity proper: it doesn't think in the same way that Daedric Princes or Aedra do. I think it's fairly well agreed upon that the Night Mother is Mephala, so it could also be that the being the Dark Brotherhood worships primarily simply doesn't exist at all, and that they (ironically, considering their strong opposition in the Morag Tong) solely worship Mephala. And there really isn't any way to consider them "good" or "noble", no. They are amoral at best, seeing as they will kill anyone if the Sacrament is performed, and at worst can be considered immoral as many of their members revel in murder.