I am not saying Sela was the best villain for Picard. And in no way is she even in the same league as Khan. You can't top Khan, IMHO. Edit: although I think Katsulas could have come pretty damn close!
JI am just saying, compared to Shinzon, and if they wanted to use a Romulan villain (besides Tomalak), she made a lot more sense to me.
What does Sela blame Picard for?
Well, her defeat in Redemption.
Her defeat in Reunification.
It could have been explained she lost a lot of status and was disgraced, so she was banished to Remus, where she raised a Reman army for a coup. Picard is dispatched, and she is bent on destroying her long time enemy. No less believable than Shinzon.
...and, most importantly, and it is twisted motive to be sure, for her being born. Remember, it was already established she was ashamed of being half-human, and hated Picard for sending Yar back in time, where she became her mother.