Well, something is the way Mala did the War, an other thing is how Revan did it.
Revan wasn't triying to crash the republic and it's army, but to take control over it, he would probably have strike Coruscant, and forced the republic to capitulate, keeping a part of it's soldiers, their military resources etc...
Unlike Malak who destroyed entire planets for his pleasure.
And also when it's say that he wanted to preserve the republic I think it's more the planets, on it, and not the political system.
But we should also think that he does fall to the dark side, not because he couldn't resist, but to conquer the republic and prepare it against the true siths. Like Kreia told the exile "maybe he became the dark lord to prevent a greater evil". It don't means he wasn't evil, it means the true Siths were worse, and that he might defeat them taking control over the republic. and then the Repubic would be better than it would be after being conquerer by the true siths.