Have you read much Batman over the years? He leans pretty hard on the Machiavellian side and is still considered a hero.
I think the difference is in that Batman rarely kills, even when he's being Machiavellian. Ozymandias arranged the death of millions, and a frame-up to boot. That's not just scope difference, but qualitative difference.
In addition, those that Bats kills are not innocents.
(And yes, I have read a lot of Batman over the decades.)
I think that the character had a high enough level of intelligence to recognize an inevitable conclusion. You don't think it was inevitable.
Arguments against the inevitability of his plan:
1) while intelligent, he failed to recognize the possibility that his plan was flawed. Despite not being omniscient, he acted as if he were. Just as he outwitted Dr. Manhattan to implement his plan, he was in turn outwitted by Rorschach, who may completely undo it.
2) As mentioned, in order to implement his plan, he both concealed it from Dr. Manhattan AND framed him for the cause. The one being who could propose and execute viable alternatives or ensure that his plan was successful beyond a few years was nullified and driven away from Earth.
3) neither Ozymandias nor Dr. Manhattan is omniscient, but Dr. Manhattan is much closer to it than Ozymandias is. Yet Dr. Manhattan didn't sacrifice himself to implement a version of Ozymandias' plan in order to stave off WW3, and was "disappointed" in the fact that Ozymandias would implement it himself. While it is true that Dr. Manhattan becomes aloof to the concerns of mere humanity, it is clear that his aloofness was a gradual development, given the story presented. At some point post gaining his powers, he still thought and acted benevolently. So arguably, given his power and knowledge, his rejection of a similar plan isn't purely a result of not wanting to help in such a plan, but also that such a plan would ultimately not achieve the desired goal.