[] The manga centers on homicide detective Hiroki Dan, the son of a former police superintendent general, and who seems to be on the fast track to career success. He maintains a hidden identity as a serial killer responsible for more than a hundred murders of people he deems beyond the reach of the law.
— justice on his own way, every chapter is a short story about different criminals
[] Park Jinsong became one of the weak, and suffered under his peers’ contempt for 10 years. However, in reality, the Edeya he had awakened was actually the S-rank “Absolute Killing Intent”. Park Jinsong was greatly disturbed by the fact that the essence of his soul revolved around the thought of killing others.
— chp 53 sudden artstyle change, mc have character development, he killed when he failed to control his power
[] "As part of a certain mission, I am assigned to teach at a magic school, and the teenagers I am assigned to teach are the students of the Class 2D, which is made up of dropouts."
While he is desperately playing the role of a teacher in order to complete his mission, a transfer student arrives and the situation takes an unexpected turn...
— an assassin that disguises as a teacher, currently not that dark
[] A hero should never kill anyone. Gangyu tried his best to do the same... until the villain who wiped out his squad killed his brother in prison. Gangyu breaks his vow and enters Ruculus with permission to kill as the prison's executioner. Now, he must fight his way through the villains to kill the worst of them all.
— have potential, still not sure how it's going to turn out
[] Lunaria il von Solfege. The protagonist, who was reincarnated as the villainess of her favourite game, was flooded with tragic memories of Lunaria. She was condemned, left alone and tried to commit suicide by stabbing herself with a dagger. *"I'll take care of everything, Lunaria. Now that I'm Lunaria, I won't give you any mercy.
— no kills for now just tortured people as revenge
[] A murderer who cannot stop the urge to kill people, even though he wants to stop, receives the death sentence he wants. His joy is short-lived, as he is reincarnated in another world. The mission entrusted to him by the goddess is to kill 12 reincarnated people using his murderous impulses. Not all reincarnated people are saints!
— he reincarnated as a woman and his mission is to kill 12 other reincarnated people that turn evil.
Brutal: Satsujin Kansatsukan no Kokuhaku