well if you think Sera's behavior is normal for mentally adult person, we must live in VERY different societies
First off. Define "normal"?
Secondly, mentally adult person??? What is that? 
I'm going to gamble and assume you're referring to "mentally healthy", or "sane"..
And going by this, let me get this straight;
You think playing practical jokes equates to insanity?
You think that telling it like it is = insanity?
You think being an individual and not acting according to what is generally "accepted" is insane?
If you're referring to the Verchiel March, and the way she beats that noble to death.. isn't that what deep down, most of us would like to do, but are held back by "laws" or "morals"?
We probably do live in similar societies.. the difference being, I don't actually give two tosses what is considered "socially acceptable". I do things my way, I'm no sheep.
Perhaps this is why I love Sera so much, and you not so much..
Alas, it does not make me a pedo, predator or a teenager. It just makes me an individual.