So Yut’s character is a full-on psycho. I think it comes as a result of three things. One, his underlying personality which must contribute to this, the long term usage of drugs for fighting, and his relationship with Sia Thep. Sia Thep makes a brief comment about Thun fighting the “replacement boy” Yut. Clearly he has always seen the talent in Thun and wanted Thun to fight for him. But apparently Pakorn’s been blocking Sia Thep for 10 years (!!) so Yut became the replacement. If, as I suspect, Sia Thep has always made it known that Yut was a placeholder in his eyes and not who he really wanted it makes sense why Yut wants everything Thun has and and wants to be thought of as the best. He doesn’t want to be the replacement, but the top dog. It also feeds directly into his hatred of Thun.
But whatever the causes, Yut has fully lost it and is straight up crazy. Also, you'd think that the fact that he needs drugs and an illegal weapon to get the best of Thun would bother him at some level (if he had any pride), but I think he is so f'ed up at this point by the drugs and being in Sia Thep's world that it no longer matters how he beats Thun, just as long as he does.
Gap does a good job with the role since I enjoyed Keen knocking him out a bit too much.