
Because he couldn't stab Jihwa so he stabbed Jihwa's low-born spy right next to Jihwa. It was a threat meant for Jihwa. Also He didn't kill the guy. He left him alive but now useless to Jihwa. An injured person is a burden. You have to spend resources taking care of them.
Now Jihwa has to take the injured and bleeding spy with him back to his estate and figure out what to do with him. You see? He's a burden to Jihwa. Plus Jihwa's servants will see the bleeding man and wonder what happened. It's further humiliation to Jihwa.
Whereas if Yoon had killed the spy he would have been a burden to Yoon and his servants. They would have to dispose of the corpse.
Again, it was highly-strategic. When the lowborn spy rushed to Jihwa's side, Yoon saw a chance to really drive home that Jihwa had messed with the wrong man. Which meant he already knew that guy was Jihwa's spy. He also wanted to make sure Jihwa took his spy with him. Stabbing the spy in the shoulder was far more effective than wasting words and saying, "Take your spy with you".
Yoon is absolutely lethal and 100 % effective in achieving his ends. I think things will end very poorly for Jihwa if he doesn't let go of this (which he won't).

Not gonna be so innocent once those pink clothes come off
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Actually, Jihwa will lose in any match-up against Yoon. Jihwa reacts by emotion and is very rash. He couldn't stay calm when the painter accidentally implied that Yoon and he had relations. I think the oranges and reds of Jihwa symbolize his fiery nature. He is very destructive.
Yoon wears cool shades of green and blue. He has a reputation for having a "temper" but only one time has he really lost his temper (when he slapped the painter). Otherwise he uses his "temper" to make people think he's unpredictable in order to manipulate them. If you read his comment in chapter one "The sight of blood just change his mind" I think he always intended to kill someone in front of the painter. I think he wanted to kill the guy he used to find the painter in order to cover his tracks. After all, that guy knew all about the painter and Yoon didn't want anyone else to know much about the kid.
Most of his threats and moves have been highly strategic. He tracked the teacher and talked him into coming to his estate. He plays the long-game.
Upon hearing Jihwa's confession of destroying the painting, Yoon did not lose his mind. He taunted Jihwa, whispered in his ears how he'd have to fulfill those rumors or the gossipers would lose face, and at the same time threatened Jihwa like a sword being unsheathed. Jihwa could do nothing but sputter in anger.
I believe pink in Korea symbolizes trust. The Painter is a very trusting person. I think the author uses the colors to represent the inner personality of each main character. Fiery, impulsive characters (Jihwa) are orange and reds. Cool, calculating, and manipulative characters are blues and green. Pink is for trust and innocence of the Painter. It gives him almost a child-like look.