If Ozymandias knew for certain that the only alternatives were (i) a nuclear war that would kill everyone on Earth and (ii) an elaborately faked extra-dimensional incursion that would unite US and USSR against a supposed common threat but that (ii) would work only if he, Ozymandias, personally killed everyone involved in the fakery to prevent them from exposing it, was he right to do (ii)? The supposed moral defence is that all the people killed by him would have been killed along with everyone else in the nuclear war anyway.
How could anyone ever know for certain that (i) and something like (ii) were the only alternatives? Ozymandias could have used his considerable wealth and influence to campaign and lobby for peace in the usual ways. Publishing the prediction of the probability of war would change the probability and effect political changes on both sides of the (in this fictitious world) still existent Iron Curtain. Ozy's elaborate plot could have failed several times and might be about to be exposed at the end of the graphic novel. Thus, there is no certainty.
That he retains his urbanity and apparent sanity while ruthlessly murdering so many suggests to me that his consciousness is fundamentally fractured, therefore completely insane.