I haven't played the game yet, but there should be somewhere else you can go leaving Broken Valley, and/or some places/quests you missed there. A ~14 to 22 level jump for opponents doesn't seem very reasonable...
The killer bunny is intended to be very tough, and triggered if you kill some number of rabbits (at least that's what it looked like in a video someone posted a link for some time ago). In the first Divinity game you could attack rabbits for a little experience (if you were at a low character level), and a bit of meat (minor healing). I think the bunny in D2:ED was meant as a part joke and part punishment for gratuitous killing of tasty defenseless creatures.
