Dear Huan Yi,
The above link is pretty straightforward.
But I think the question you raise has to do with the sheer flexibility that conscience is capable of contorting itself into and how astonishing that somebody with Stalin's initial ideals could end up being such a freaking monster without apparently even losing a night's sleep over any of it in the process. I have had the same problem.
I've noticed that almost everybody seems to give themselves what I think of as "the protagonist's discount." Because we are all at the center of our own stories, we are essentially our own heroes, and because we are the heroes, by definition, we tend to give ourselves the hero's favored place. We root for the hero. We hope the hero wins, and, probably more than anything, we assume the hero is the good guy.
Arrant twaddle, of course, when you examine the notion from any sort of objective stance; but that's the beauty of the protagonist's discount. We don't charge him as much when it comes time to hold him accountable for the consequences of his actions because, as Pogo might say, He are us. And we know we always mean well and we are, without much doubt, well-meaning good guys.
If even fairly regular folks are easily felled by this delusion, why wouldn't a full blown paranoid psychopath like Stalin be even more easily deluded?
I mean, if you can get away with calling yourself "Joe Steel" as a nom de Guerre without crumpling into gales of self derisive laughter and still somehow manage to take yourself seriously, what kind of claim on real sanity do you really think you have?
I'm not sure if this was more the sort of reaction you were looking for, but I was fascinated by your question and I have been eyeing the Stalin book with some interest myself, so I thought I'd try giving a shot to a reply?