[To Leia, Obi-Wan's attempts at discussing this in a more vague sense, talking about anyone being capable of great good or great evil, is just a stalling attempt. She wants an actual explanation, not a discussion about the complex natures of humans. And as far as she's concerned, even though he spared Luke, all Anakin is capable of is great evil.]

To be influenced by someone to that extent, there had to be something resembling those feelings there to begin with.

[which she's convinced of, and partially why she's uncomfortable with Anakin himself. Luke may have seen the light in his darkness, but looking at him here, all Leia sees is the looming darkness underneath whatever light he wants to pretend he has.]