@KaumGao it's a combination of character development... and the fact that he's becoming jaded. For example, take an innocent child who gets kidnapped by some psycho, who ends up killing his kidnapper to escape and survive. That child isn't going to be so innocent anymore, and no matter how strongly good moralistic ideals are stuck in his head and heart, his perspective has changed - he now knows that life is fragile. That even his tiny self can kill a scary adult. That sort of thing is going to change the way you see things, the way you look at them, and alter your choices in the future. Take that same kid, fast foward to high school. Maybe he's stil a scrawny nobody, but say he gets caught up by bullies who want to beat hiim up and steal his money or something. He already killed someone larger and more terrifying than those kids - so instead of thinking "do i run, or do i just give them my money and hope they don't hurt me too much" the options he's going to be thinking of will be far more likely to include "do i fight back? do i fight back enough to run, or should i try to kill these jerks?". As time goes by and one encounters new situations and hardships, more and more of these doors and possibilities are going to open up, because the line was already crossed.
as another example: most kids, especially boys, grow up with a fascination for guns, knives, and fire. Obviously most kids aren't allowed to get their hands on any of these things. Now, the average kid isn't going to suddenly burn their house down. But that kid who finds a lighter to play with? Chances are he's going to use it. Burning leaves and sticks, or pieces of paper, or whatever else he can find. Maybe a bottle of grilling lighter fluid gets swiped from cupboard. And once you've played with burning one thing, you'll be far more likely to try burning other things. Maybe insects, just to see the way they shrivel up and pop. Maybe the hair on your arm, after spraying it with aerosol deodorant. Maybe a house, or a school. And then, after all that, if the kid doesn't get stuck in juvie, and isn't traumatized by getting caught in a burning building - what are the odds of that kid being a true pyro? How likely are they to try starting bigger fires in new places?
Our mc here is doing just that. Each new mission is filled with situations that force him to do things like kill people and make hard decisions, and to open up his perspective and break the limits of his morals, in order to survive and succeed. Each time, he can go a little farther, a little harder, and making cruel decisions becomes easier. Eventually, he's going to reach the point where the morals don't even matter - just the efficiency. Who needs to die to make this mission succeed? Who needs to be saved? Who should he lie to, who should he confide in, who should he manipulate to what end... etc.
This story is doing a fairly decent job of exploring this development. He used to be a completely normal guy. Scared of fighting, scared to die, scared to kill. But he's been forced to do things that change him - and the best way we can judge him isn't by what he does exactly, but why he does it. How is he going to balance unsympathetic efficiency with his own sense of morals? Where will he draw the lines? Would he kill innocents to gain success, or will he choose to suffer further hardships to maintain his dignity and sense of honor and humanity?
Right now he's just doing missions for money, because it's a job that pays damn well with some fairly high risks. He's doing what he can to maintain his integrity, even after being placed in the shoes of murderers and betrayers and forced to empathize with souls filled to the brim with hate, to the point they literally sold their soul just to live through a repeating illusion to see how the things they regretted most could have gone differently. But how long is he going to keep that up? He's barely tipped his toes into this business, and it's pretty obvious that things get harder and darker the deeper he goes.
