The MC, who upgrading by killing people, that is, for example, for 1 kill, +5 strength is given, for 10 kills, a new ability is given. And the MC should not be afraid to kill, that is, this should be his main way of upgrading. P.S. It is also possible for experience. That is, for killing 1 person, 10 XP is given, this can generally be a way of cultivation. The main thing is that for the MC, killing people is the only way to develop throughout the book and he is not afraid to kill.