I think Chuck threatened hwayoung and although he’s capable of handling himself and he has the backing of his family, he doesn’t want his lover and master to be hurt. He did it this way so hwayoung wouldn’t interfere until all of it was over. What I don’t get is why he didn’t just leave in the dead of night, go to Chuck, say he broke up and then shoot him with the gun. He knew Chuck was gonna give him a weapon. Why didn’t he make a plan involving killing Chuck? It’s not like it’s the first time he’s killed someone anyway.
If I'm not wrong, there were other people out there, besides chuck, are supposed/able to force Gyuwon into participating in the mission. So, if Gyuwon were to do that, there'd be a huge chance that Hwayoung will get more endangered. He and Chuck also go a long way back at least that's what it seems in my perspective, hence, even if he did thought about that; he'd hesitate.
But he hates chuck. Going way back could mean anything. I go way back with my mother and yet I hate her guts. I do agree that it could be due to others but the reason they want gyuwon is because he’s the strongest of those there so why would they go against him if he’s got a gun and is already on a shooting spree? Also as far as I know, only Chuck is aware of hwayoungs significance and existence. He even said “after this is done, you can go back to him” so it’s not like he has any investment in whether or not gyuwon is in a relationship. He just doesn’t want gyuwon distracted. So my point still stands. Why did the breakup have to be real? Why not just kill Chuck and make an example of him? That’s the merc way to begin with.
Aside from safety razors not having those blades, I don't get why he couldn't just talk to hwayoung and tell him this was something he needed to go do, and that the moment it was over he would return and explain everything.
The lack of communication and choosing to be hurtful by leaving him is just disrespectful to hwayoung and to the relationship on the whole. D/S is built of trust and communication. I really don't get this point of the plot.