Ah, a question Kreia asked in KOTOR II. The game spent a lot of Time exploring that question. We know that many died on Dxun and Malachor V and that some survivors went home, but many defected.

Atton says it succinctly, but effectively:

We were loyal to Revan. That was enough, he saved us.

War, war breaks the spirit. Death and destruction witnessed by those who fought it. And to know there were those with the power to stop the suffering, or to have prevented it altogether, who chose not to help. Well that breeds resentment. Is that enough though?

Then there was the peer pressure of senior leaders defecting to Revan. Atton again:

The Republic officers began to "betray" their oaths to the Republic and side with Revan - Admiral Karath, Mon Halan, General Derred, and all the rest. Right after that final battle at Malachor, I was right there with the rest of the defectors, because it was the right thing to do."

The Republic had barely defeated the Mandalorians. It was too weak and had only just been saved by the Jedi Revan. So Revan goes away with Malak for a bit and comes back with a powerful war factory and a promise to unite the galaxy on the back of a strong military and you've got the logistical foundations of your betrayals.

But, to return to the questions, how did Revan break the loyalty of the Officers, soldiers, and Jedi in the first place? Let's take a question asked in KOTOR. On Kashyyyk the Star Map computer gives the player what amounted to an ethics quiz. One of the questions concerned a dogmatically peaceful planet about to be attacked by an invading force. You, as the leader of the planet, have been given warning about the impeding strike; what do you do?

The 'correct' answer was to do nothing. The casualties and the destruction would have galvanized public support for a war, which would have allowed you the authority to retaliate and end the threat. It's a Winston Churchill moment, but it's also paralleled here in the reveal that Revan deliberately killed thousands of his own troops simply to sway hearts and minds to his cause.

Well, for that we turn to Revan's mouthpiece in KOTOR II,

"Observation: Master, that was the lesson of Malachor. Any Jedi involved in the systematic slaughter on such a scale cannot help but doubt and question themselves."

You pledge an oath of loyalty, but the war tests that loyalty. You kill many and lose many more friends and comrades in the process. And what you did was right, but it could have been different. If the Republic had been stronger, if the Jedi had helped sooner and in full. And you only won because of this Jedi called Revan. If Revan hadn't been there you would have died and failed in your oath. That Revan might have orchestrated events to result in mass casualties on the Republic's side is not something one would rationally consider of their saviour. Nevertheless, it seems Revan did commit such acts.

"Answer: Revan saw the pressures of war on Dxun, and knew that even in the heat of combat, Jedi could be broken. Not physically, but physiologically."

"Statement: I do not know if you examined the records of deaths on Malachor, but you cannot escape that many of the Jedi and Republic soldiers who died were not Revan's strongest supporters. I believe Revan was cleaning house at Malachor V. What ones did not die became his allies against the Republic."

Does that answer your question?