Revan was corrupted before s/he discovered the maps leading to the Star Forge. Malachor V was a Sith academy and it is where Revan supposedly learned about the Ancient Rakatan technology that was powerful in the dark side. So s/he probably corrupted Jedi there.
Atton said "The soldiers were loyal to the Jedi, not the ones that sat back and watched others suffer, but the ones that saved the Republic"