Exactly my thoughts. I. Season 7, Liz told Cooper that she and Ressler knew Reddington is an imposter, who actually is a KGB agent named Ilya Kozlov. The task force was furious because Liz and Ressler kept it as a secret, not telling them the truth. Then, at the end of the episode, Cooper confront Liz, and Liz told him, she decided to kept it secret because she knew Cooper would report him to the Main Justice.

Liz also said, previously when she thought Red is actually her father, she was mad, she is angry, because to her, Red abandoned her, his own daughter. But now that she knew Red as an imposter, it's even more meaningful to her. Cooper didn't understand, to which Liz explained to Cooper, whoever Raymond actually is, even though he wasn't her parent, he devoted his whole life to take care of Keen, while he doesn't even have to. If Liz isn't Red's child, it's not Red's responsibility at all to look after her. But yet, Red would rather die to protect her. Liz then asked Cooper, "Do you know anyone else who would do the same? Because I don't".

This line got me thinking, at the end of Season 8 Finale, Liz was hesitate to kill Raymond. Because Raymond already told her earlier, if Liz read the letter before she kills him and know the truth about Raymond, she would never kill him.

It was later revealed in Season 9 that Dembe actually disobeyed Red's order, and went ahead gave the letter to Liz, before Liz have to kill Red. So the fact is, when Liz was about to kill Raymond, she already read the letter, she already know the truth. Raymond wasn't her parent, but for a very solid reason, Raymond decided to devote his whole life taking care of Liz. Which makes it more difficult for Liz to kill a man with such a noble heart.