What you're asking basically forms the foundation of genocide studies. To summarise a great deal of the field and the work therein (citing broadly Kieran, Totten, and for better or worse, Israel Charny) - Hitler, Stalin etc. not only considered what they were doing to be okay, but saw it as absolutely necessary.

There is a kind of a tipping point that makes most genocide extant (I'm leaving out cultural genocide because that's a much trickier ball to play with). The point comes when the group (Group A) that identified another group which was previously identified as a problem/threat/drain/vampire/corrupter etc. (Group B) becomes convinced that Group A's very existence is now threatened by the continued presence or existence of Group B.

You can extend this to almost any genocide in which murderous atrocities are committed. Hitler didn't start killing Jews, Gypsies, Homosexuals etc. en masse until Germany started losing the war. Certainly there was still disgusting amounts of persecution, but the industrial level killings had not started. Rwanda functioned in much the same way, only it was a far more planned out and sickeningly more effectively executed genocide - fear that the end was coming drove the destruction.

I want to take one moment to oddly defend Mao, whom I thoroughly dislike as a leader and think Deng was in every a better leader. While Mao presided over the Great Leap Forward, which unquestionably killed untold millions, there is actually very little evidence that he intended anything of the sort. What I mean is that most of the available evidence (that I've seen, obviously) points more to Mao being blind to the fact that his officers were fudging numbers and lying to him out of fear. This does not in any way take away from the tragedy, but whereas Stalin, Hitler, Pol Pot, Milosevic et al. clearly intended to kill or wipe out great swathes of people, Mao in this instance comes across more as a vain idiot than a murderous brute (which he clearly was, just not here in my opinion). Further, even if Mao intended millions of people to die, it is nonetheless true that he wanted to make China stronger for it, just as Hitler, Stalin and others believed their respective genocides would make their respective peoples stronger.

TL;DR - Not only did they think it was okay, they thought it was necessary for their people's survival.