I would argue that Marxism's "meta" (as you said, theory of human nature, theory of history, theory about what's socially important, but also economic theory, etc.) is significantly more destructive than mere socialism, the reason being that practical socialism can, with certain qualifications and in limited application, be quite functional, whereas Marxist theory is more universally destructive to human ability and spirit (which is why I think that, had the more "pure" and "orthodox" Trotskyism prevailed over Stalinism in the early Soviet Union, the consequences would have been even more disatrous, at least in the short term).

And I agree that Hitchens is a brilliant crafter of words, though God Is Not Great might have benefited from more research and less rhetorics. For all its fire and bluster, it is riddled with inaccuracies, misunderstandings and plain false information. It's an opinion piece, and I suppose that's what he was going for, but too many people unfortunately seem to treat it as a scholarly work, which it most certainly is not.

In fact, this may be a common element in Hitchen's approach to topics as diverse as Marxism and religion: He knows what he'd like to say about the idea very well, before he knows the idea very well.