Greatest doesn't necessarily mean power. After all, Palpatine is the greatest Sith Lord for accomplishing more then the rest of them, but that doesn't make him the most powerful by default.
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.
"You are greater than any I have ever trained. By killing me here - you have rewarded me more than you can possibly know. And Revan knew that the true war is not against the Republic. It waits for us beyond the Outer Rim. And he has gone to fight it, in his own way. He left the Ebon Hawk and its machines behind, for he knew he would not need them. And, like you, he knew he must leave all loves behind as well, no matter how deeply one cares for them. Because such attachments are not the way of the Jedi, and they would only bring doom to them both in the dark places where he now walks. Perhaps you shall go there with him, and do battle at the end of all things. Instead, I remained here... and now show others the way."
It could mean many things. Kreia wanted the force to die, she hated it. When she saw the death of the force inside the Exile, she loved him more than anyone almost. By greater, she could possibly mean that he was greater to her, because he could do exactly what she dreamed off...
It could also mean that he was more powerfull than anyone else. He stole power from everyone that he killed and drained his friends force powers as well.
The instakill could also have been done through the Exile, as Kreia's quote "see it through the eyes of the Exile", then the 3 jedi masters are drained of the force and dies, just as the Exile is "drained of the force". Also, Kreia's instakill might not work on the Exile because it drains the force, and he has no force....
Some say that it does not drain people like Nihilus ability, but it does. Just walk up to the bodies of the jedi masters, it says they are drained and worse than lifeless. Then look at the beam that Nihilus produces while draning the Exile, and look at the beam that Kreia creates to drain the masters, it looks exactly the same, the sound as well...
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Necessity is the mother of inventions.
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