I noticed on my recent read through while thinking to himself Fain says he tainted her. I think he uses the term that he planted a little seed with Elaida. Since it is before he had the dagger back in his possession, it was not much, but enough that he thinks on it.

I personally think Elaida has always been this way. When she first meets Rand she wants to put him in a cell and to question. She is power hungry and willing to torture Siuan for her information and is responsible for the death of Siuan's warder so she could have her position. This was before she even met Fain. She is a hard person with no concern for others. Even though she does not support the DO, I think she is evil. She certainly does not have any kindness in her. It does not take much from Fain to push her down her path.