By analogy, you could say that the Inquisition was inherently unchristian, in so far as Christianity has an ideological character beyond simply "Christ worship".
Similarly, if we do the obvious thing and conflate "left wing" and "socialist", and believe that socialism has an ideological character beyond simply hating the existing organization of society, then extermination, slavery, thuggery, coercion, and violence are inherently anti-socialist.
This view is also the conclusion Orwell was operating under---the utopian ideal of capital-S Socialism which he approved of was used as operating cover to assist in the seizing of power by technocratic middle classes.
This view was pretty much explicitly stated in the Goldstein treatises in 1984, which described "English Socialism" as actually a form of "oligarchical collectivism," and claimed that "The Party rejects and vilifies every principle for which the Socialist movement originally stood, and it does so in the name of Socialism."
Similarly, there's this bit from the supposedly ex-Trotskyist James Burnham (who Orwell rightly abuses for being a power-worshiping scumbag):
"Some apologists try to excuse Marxism by saying that it has ‘never had a chance’. This is far from the truth. Marxism and the Marxist parties have had dozens of chances. In Russia, a Marxist party took power. Within a short time it abandoned Socialism; if not in words, at any rate in the effect of its actions. In most European nations there were during the last months of the first world war and the years immediately thereafter, social crises which left a wide-open door for the Marxist parties: without exception they proved unable to take and hold power. In a large number of countries — Germany, Denmark, Norway, Sweden, Austria, England, Australia, New Zealand, Spain, France — the reformist Marxist parties have administered the governments, and have uniformly failed to introduce Socialism or make any genuine step towards Socialism... These parties have, in practice, at every historical test — and there have been many — either failed Socialism or abandoned it. This is the fact which neither the bitterest foe nor the most ardent friend of Socialism can erase. This fact does not, as some think, prove anything about the moral quality of the Socialist ideal. But it does constitute unblinkable evidence that, whatever its moral quality, Socialism is not going to come."
To be fair, it's easy to believe claims about Stalin not being a socialist are simply ego-bruised leftists invoking the No True Scotsman fallacy. I don't think this necessarily applies simply because socialism is inherently an ideology. If someone claims to be a pacifist while marauding through a public place with an assault rifle, massacring people as they go, we have no problem resolving this dissonance: the murderer's claims of pacifism are simply lies.
Of course, pacifism was never taken all that seriously to begin with, so it's safe for us to simply say "you're lying about being a pacifist." We feel a bit more constrained telling someone they're lying about their status as a Christian or a socialist.