It's not revenge. The synopsis said they have feelings for each other though it was never shown in the past. And now you can see the image in Seo-In's eyes of how much he wants YeongWoon. And YeongWoon had that dream about Seo-In as well after having seen HaeYool w/ him (when they met up privately at the hotel room 702) and also in his thinking process that if he goes to work for Seo-In and rekindle that relationship that they would end up going beyond the friendship, crossing the line of no return... You can see HOW MUCH they THINK about each other in their thought bubbles and the intensity of their interactions...
It’s a revenge story isn’t it and the Protagonist isn’t going to end up happy right :/