Oh well...I have to introduce the term "freedom" here or that what Kant would call a "causality of freedom".
The whole question is not a problem of conscience. The question is not what you should do the question is what you personally want to do. And you can't answer the question what you want to do unless you have done it. It's not your identity that determines your actions - it's your actions that determine your identity.
Now you have to see the action you have to take out of context. Do you want to kill the child ? Hell...no...Do you have to kill the child because this is the only way to save millions of people ? Probably yes in the situation you created. And in this case the freedom of choice you have might force you to take an action and thereby it will determine your character.
That means: Killing the child might be seen as "right" if you view it from a point of necessity but it can never seen as being the moral right thing to do neither for you (because you can't say: "I'm proud because I've killed the child !"😉 nor for the public or lets say the people you have saved because they will know that to save their lives you had to kill an innocent baby.
Now...to come back to Revan again. What Revan did...was it "right" ? Was it necessary ? Was it good ? Hell...no.
He killed many Jedi, he killed innocent people, he assassinated politicians to do what - strengthen the Republic ? What kind of thinking is this ? That's like saying (Janus gave the example) "If I am the ruler of the world their will be no wars anymore because there is no opposition to fight" or - more exaggerated - "If I kill all human beings there would be no criminality any longer".
He was neither acting because of a moral righteousness nor out of necessity so what he did can only be considered as being wrong. And that's why he takes the "right" action (from a moral point of view) past KotoR and goes to fight the Sith Empire on his own - without using / risking / taking the lives of others like he did it before.