 If you liked
Zankoku na Douwatachi
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 ...then you might like
Die
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Both are mangas by Kaori Yuki, so you can expect great art, strong story, some level of cruelty towards characters and psychological plot. What else could you ask for? :) Die is oneshot which contains only about 16 pages, but it still has stronger story than most of extremely popular mangas. Zankoku na Douwa-tachi is collection of three oneshots. My personal favourite is Cruel Fairytales. I highly recommend them, as well as other masterpieces of Yuki-sama.