Actually when Revan first turned to the darkside he/she refused to accept it. He/she believed that by turning to "Sith teachings" he/she could become more powerful than he/she could as a Jedi and gain the power needed to stop whatever dark force was looming in the Unknown Regions. That was he/she was doing was actually "for the Republic".
At the beginning perhaps it was. But honorable intentions don't justify the means Revan took and they lead him/her down a far darker path. If you read all the books Revan completely fell to the darkside when he/she first stepped foot on Malachor V. Revan realized he/she had actually fallen to the darkside, had completely embraced the Sith teachings... and so the fall was complete. Revan then became the conqueror of the Republic, not its hero.
If he/she had wanted to help the Republic he/she wouldn't have done the things she did, wouldn't have killed all those people, sacked all those worlds. It doesn't add up. Revan actually had Malak initate a secret program where they trained assassins to turn Jedi or kill them in horrible ways.
Does that sound like he/she was trying to strengthen the Republic? No. In the end he/she wanted to destroy the Republic. And canon agrees with me. 
