As I've mentioned a few times, I just find the whole claim and the subsequent stalling to be really suspect.

If she really had something earth-shattering to tell us, she wouldn't have waited so long to tell us. Apparently she had hinted at something at the outset of the Day, but no one took it seriously. Now she reveals that she was the beast who attacked Napoleon... and that's all she's going to say about it. As beet mentioned already, it wouldn't take long to put out the important info/rationale for whatever she did, but she hasn't.

At this point, I wouldn't be totally shocked to learn that she is neither a beast, nor the person who attacked Napoleon last night (though I doubt we'll hear that from her herself, but possibly post-game/death if my hunch is correct).

So, for now, I propose shelving the idea and focusing on the vote thief issue and whether Napoleon was attacked, who really attacked him, etc. I mean, if Yuna is a Jester, she couldn't have attacked him, right? I've never heard of Jester Serial Killer.