it was the same to me too. When I read it the first time, I thought the kiss came out from nowhere lol. However, when I re-read it again and paid attention to Taehee's actions rather than his words (and the plot), it does make sense.
For example, just in the very first encounter, Taehee was like "fuck personal distance", he was so close so close to Jaegyeom physically (and he didn't even know anything about Jaegyeom yet). He was already attracted to Jaegyeom.
2nd time encounter when he helped the tie, HE STROKED JAEGYEOM'S NECK, HE ASKED JAEGYEOM TO LIFT HIS HEAD UP. I realized how invested I was for the plot when I read it the first time, because how I didn't see the sexual intense in the first time!!!
There are 2 things in the novel that didn't get in into the manhwa. When Taehee called Saero to help with the blood, Saero intended to get more blood from Jaegyeom by any means. Taehee said "but I don't want him to get hurt"!!!! And when Jaegyeom found out Taehee a gwijae, Jaegyeom said he'll break Taehee's wrist if he not let go. Taehee's inner monologue at that time was "if I am able to keep you, a broken wrist is nothing"!!!!
Then come to the play "if you get physically close, the hearts will follow" LIKE BRO!!!!
Almost die, in the middle of a fight, "do you hate me this much? If I quit being an exorcist, will you like me then?". Uhm sir wth are you doing???
He visits Jaegyeom when his arm still in cast lol. He caressed Jaegyeom's face, cheek and lips!!!! When he sees Jaegyeom cries in dreams, he couldn't bear to see it and he called Heukje out to make sure Jaegyeom not seeing nightmares anymore.
He keeps teasing and flirting to Jaegyeom. The tease the flirt you ask? "let makes today my birthday because you go out with me" "I was in a fight of 1:6 and I was 1 of the 6. I almost die but the opponent was kind-hearted, I made it out alive"
He expresses to everyone what Jaegyeom means to him. He lets all the new orientees know that Jaegyeom's backer is the famous Yoon Taehee (so nobody will mess with Jaegyeom) He told team 1 Jaegyeom is a precious person that he went to hard time to get so everybody please nice to Jaegyeom. He told Seok Juryeon that it's a no for the other kid touch his subordinate. He lets Jaegyeom and his family leave in a big big house (in the novel, he even purposely picks a good date for the family to move). All of that mean he's really nice and caring to Jaegyeom ~
and they are nothing related to the revenge plan lol
More and more physically close scenes. one in the elevator, one when he ties Jaegyeom in the fitting room, another one when they outside the restaurant he falls asleep on Jaegyeom's shoulder after "maybe he'll run away with the bride"
The whole situation is very well-built imo. In the deal, their hands weren't shaking but they place on each other's neck. Jaegyeom's death will bring Taehee's success. And Jaegyeom's only wish is the end of his cursed life so Yoon Taehee can only win. Just think about it, in 2 months, the only one who shares your secret, the one who joins your to the rebellion, the one "it doesn't have to be you, but I want it to be you", will disappear. Taehee has sense this that's why he wants to break the balance between him and Jaegyeom.
Also, look at Taehee's life. He doesn't have any friends. The ghosts are more like his subordinates. he will eventually betraye all the so-called 10-year coworkers. His life has been only revenge. Only Jaegyeom, the one he can treat as someone equal. Jaegyeom is just very special to him.
I think if it was other BLs with those above, I would giggled and kicking the blanker already. However, I was so invested in the plot and Taehee's revenge plan, same as him I didn't realize his actions screaming out loud that he has fallen too deep.
Sorry for the long post!!!

i just finished binging all the available chapters and the plot is super intriguing but idk if its just me but i feel like theres something lacking with the romance aspect of the story TT idk if its cos the pacing of the manhwa differs a lot from the novel but i feel the tension between the two main characters with the blushing and ig sexual tension just feels so random