Never read Battle Royale, only seen the movie, but I don't see the correlation between that and Berserk. Unless you just mean dark, violent manga in general.
Berserk isn't all grimdark though. Its major theme is about humanity and the struggle and will to survive.
I got through like 11 or so volumes some years back, always meant to keep reading—except now it's been so long, and with the release of the deluxe oversized editions, I decided to start over lol.
Some of the most amazing manga art, if not THE best, in my opinion. Kind of a cross between western and manga style art. Miura was insanely talented, and really had a unique vision and storytelling sense. Also, like any good artist, he didn't try to hide his influences (Verhoeven's Flesh+Blood, Hellraiser, the phantom from Phantom of the Paradise—even if these were all pretty superficial pop culture references, it was still cool to see what he'd borrow).