In Dune and subsequent books we learn that Gurney and Duncan had been freed from Harkonnen slave camps etc and pledged loyalty to House Atreides. Did the Atreides see themselves as freedom fighting liberators of the defenceless, rescuing people from unjust treatment or was it just a personal battle against House Harkonnen? Given that Duncan Idaho was the best man-at-arms in the universe were the Atreides using all means to improve their military status? (Sorry if this is a new/different question, I will remove if so).
In the days of the Corrino Empire, there is some reason to believe that the Atreides did take what we, in our modern day point of views, would consider a more progressive view of human life. This is not to say the scrupled to kill (Thufir Hawat was Master of Assassins) when they thought it necessary, or that Caladan was ruled by anything we'd recognize as democratic means (Arrakis certainly wasn't, even under Duke Leto, but then, during the transition, it was essentially martial law). The Empire, as a whole, accepted as given the idea of a caste system akin to the European middle ages, and the Atreides found it just as odd that the Fremen held themselves aloof from that system as anyone else.
That said, we also have no indication that the Atreides routinely went out to free other houses' slaves, or were seen as abolitionists. In fact, the only sense in which the Atreides are seen as being exceptional at all, within the context of Imperial politics, is that Duke Leto was personally popular, despite not being very wealthy or powerful, and therefore a potential threat to an emperor who had no sons, in a culture of male primogeniture.
Similarly, we see no indication that anyone else in the Imperium finds it odd that the Harkonnen have slave labour, or that Harkonnen government, per se, is all that out of the ordinary. Only that Vladimir Harkonnen himself is unpopular and ruthless in the pursuit of his family's interests. No one condemns him for slavery; they condemn him for being treacherous in his dealings with his peers.
So while it's impossible to say with certainty, textual evidence suggests that the Corrino-era Atreides are just another Imperial family with an agenda, whose leadership we find more sympathetic because they tend to be somewhat less underhanded in their dealings, and don't appear to keep slaves themselves; rather than active champions of liberty.