There is another possibility.
1) Kreia is speaking literally about a Darth Traya always existing in some form or another.
2) She made plans to become a dark side force ghost in case Exile defeated her, and thus plans to survive her own death, and remain as Darth Traya.
3) As a force ghost/betrayer, she could already be planning to betray the next generation by misleading some other poor sap.
"If force is the game, the murderer wins over the pickpocket." Ayn Rand
"Justice is the midpoint between being treated unjustly, and treating others unjustly." Aristotle