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.
So the kills won't be for XP, but directly for stats? This would be way too OP, especially once they get a decent AOE and finds a crowd of people.
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.
Profane Prince of Domination's MC has this during the mid to late stage of the story. Befoee he only has sex. Now both sex and murder empowers him.
There is something that kind of work like that but it may or may not be what you want "Leveling Murderer – If I Don’t Kill Someone Once a Week, I’m Going To Die, so I’d Rather Do Good Than Evil~" XD