The closest thing to Nirai Nikki is probably
Deadman wonderland the romance is there but you have to wait for it. everything else is pretty much the same except the setting.
http://myanimelist.net/manga/3986/Deadman_Wonderland
the spoiler you put up for death note happens but is not the real ending.with Karin you have to be patient for 5 episodes after that it gets interesting.