Responses
Taekjoo just underestimated Zhenya. He wasn't dumb.
And he didn't go back for revenge per se, but to finish the job, that he came to Russia for in the first place. (If he hadn't come up with this plan, his real 'partner' would have shot him, before he would have ever gotten the chance to go back to Korea.) Getting back at Zhenya, while doing that, was just a bonus, not the original goal.
This is why he was always the dumb one in those tiktok vids with him, leewon and taekjoo