Revan's childhood, and the origin of his philosophy.
When he was just a kid, Revan was scouted by the Jedi Order. And just like all children with the potential to use the Force, he was taken to be trained as a Jedi. Now, one would call this a form of child kidnapping, but since the Jedi were supposed to be just and righteous, no one would question their methods. At least, not yet. Many things would change, however, when Revan grew up, as he would be a highly influential figure with interesting ideas and morals, ones that would challenge both Jedi and Sith. But what kind of life did he experience to acquire his philosophy?
After being introduced to the Jedi Order, Revan underwent the standard Jedi training. Lightsaber training, using the Force, learning the history of the Jedi, etc. The standard training for all Jedi padawans. Revan got along well with all the other padawans, but if he were to choose one of his friends as a best friend, he would pick Bastila Shan. Bastila was an interesting girl that exhibited more emotion than a Jedi should. The first line of the Jedi Code stated "There is no emotion, there is only peace" which seemed a bit odd. Revan and Bastila often found themselves quietly talking about the Jedi Code and its hypocrisy, taking care to make sure none of the others found out.
The small talks with his best friend is where Revan started making ideas, philosophies. Emotions are what separates living beings and robots. Sometimes, being told that Jedi had to fight for the safety of the galaxy sounded more like "Jedi are trained to be emotionless, mindless, robot-like killers that would kill enemies of the Jedi Republic." The enemies were never specified beyond "Sith Lords usually have red lightsabers, and are evil." leading Revan and Bastila to realize that the instructors were their only source of information, and that they had no idea whether or not their teaching were actually correct.
Revan also wondered how he ended up in the Jedi Temple. Bastila faintly remembered that she had biological parents that basically handed her over to the Jedi, but Revan remembered nothing. When asking one of the older masters, they would simply say "I'm sorry, young one, but you were taken at a young age for a reason. Emotional attachments are against the Jedi Code, so it was probably better that you had no attachments to your parents."
Revan found that emotional attachments actually strengthened him, and that they weren't as corrupt as the Jedi Masters claimed. Revan never felt the so-called "pull to the dark side", and he was good friends with Bastila for nearly his entire childhood, which was about a few years. Revan saw nothing wrong with friendly love, compassion, and other good emotions like happiness. If anything, it gave him good motivation, and Revan turned out to be one of the best Jedi ever.
Revan continued his life as a Jedi padawan. The only reason he didn't quit was because he really had nowhere to go, and he doubted that the Jedi Masters would let a Force-user slip from their grasp. The padawans started getting field missions, for experience, which separated Revan and Bastila. They rarely saw each other for the next few years, though they did chat through holographic calls, making sure to avoid subjects that would lead to the breaking of the Jedi Code through logic. You never know who is listening, after all.
More years passed. There was nothing eventful, besides the occasional battle as a Jedi padawan, until the Mandalorians started attacking. That's when Revan began to think that the Jedi were no so righteous.
"What?" a slightly angered Revan asked. "There is no need for us to interfere. Our job as Jedi is to protect the innocent and keep peace, not fight military wars. That is for the Republic." Revan simply stared at the Jedi council members. He was pretty much a young adult now, and after finding about the Jedi Council's decision to stay out of the war, Revan, Bastila, and a group of people with similar thoughts had decided to question the Jedi Council.
"At the rate the Mandalorians are going, they will throw the entire Republic into chaos with all-out war. They only have one or two systems now, but they'll definitely continue to conquer. More conquest means more war, which definitely isn't peace, and millions of innocents are dying in the crossfire. Staying out of the war would not be keeping peace or saving innocents." Some of the council members considered his words, but they were stubborn, and did not like their ways being questioned. "We feel sorrow for those that have perished, but-" one council member tried, but Revan had enough.
"How can you feel sorrow when the first line of the Jedi Code has the words 'There is no emotion, there is only peace.'? And now that I think about it, why are emotions bad? Sure, anger and hate may be negative emotions, but the positive emotions are good as well. There is nothing wrong with feeling compassion, or happiness, or-"
"That is enough, padawan. Such dangerous thinking will lead you down to the path of the dark side." Revan internally scowled, thought he did not let it show. "And your thinking will result in a war which will cause the lives of millions. At least we can shut them down before they can kill more innocents, so why are we sitting here, defying the Jedi moral of defending the innocent?"
Revan was met with no answer. Some seemed to finally realize that there was something wrong with current methods. However, Revan had kept his thoughts to himself for 15, almost 16 years of doing mundane tasks that didn't so much as protect the innocent as much as protect Republic assets. He endured the Jedi for a long time, but now he was done.
"I have asked you questions, and you failed to respond with satisfactory answers. I believe that you are nothing more than hypocrites that fail to follow what you teach. If you refuse to see the truth, then I will do what I must, to protect the innocent."
None of the Jedi stopped Revan from leaving, nor did they do anything when they received reports about a group of Jedi led by Revan entering the war. His philosophy had stunned them, rendering them unable to think clearly. The Jedi Order didn't matter anymore, though, as Revan carried his ideals into the Mandalorian Wars.