Hey guys, I recommend Ijimekaeshi. I am surprised that I havent seen anyone post this here before. Ijimekaeshi is a manga about a girl that wants to become an idol, but after joining an agency, she gets bullied, tormented and tortured by the other idols. One day, she had enough and tried to fight back, which results in the other idols trying to kill her. She defends herself and finds out how good it feels to hurt her tormentors. The whole manga is full with cute evil girls getting killed in all sorts of way. They are also ecchi/yuri scenes, and the kills get more brutal as the story moves on.
Here is the first revenge kill (EDIT, I numbered the pages 1 to 13, but for some reason they are not in the correct order):