Well, hello! Long time, no see, but I can’t resist a good question.
I think there’s more to Beatrice’s motives than meets the eye. That goes without saying, in a mystery like this one – but in another thread @UsagiTenpura pointed out Battler is really fighting to prove Beatrice’s innocence. Which means, for all her cruelty and bluster, the Beatrice in the Meta World is innocent of the crimes committed on Rokkenjima Island.
I think Meta Beatrice wants Battler to win, but more than that, I think she wants Battler to understand her. She wants Battler to find out who she is, what she is, and what happened on Rokkenjima. The words of the bottle message, even if they’re written by an imposter, may reflect her true thoughts. ‘You who have read this, please find out the truth. That’s my only wish.’
Why not just tell him, then? Well … that’s a more complicated question. It could be because Meta Beatrice, Beatrice-the-Witch, isn’t real, exactly – she seems to be influenced by other people’s perceptions of her. It could be because Battler needs to find the answer himself. But she does get him fired up, and the crueler she is to him, the harder he tries to show that kind of cruelty is impossible. The Witch of Certainty keeps telling us that Beatrice is a good actress, so maybe we should believe her.