I think it's more that he ended up in a "hostage" situation. Technically, he begged Guwon to take him in and he's accepted. But then Guwon killed his family in an instant. What would happen if the vet immediately went back on his words?
Guwon easily overpower him, the vet is scrawny and they live in an apocalypse world. If the vet reacts aggressively to Guwon, he would die. If the vet runs away, he has nowhere to go and can get eaten. Technically, the vet has no choice.
Also, what Guwon did was just absurd. I imagine if I was tue vet, I probably be in shock/stunned cuz what he's doing just doesn't make sense (morally) and so unpredictable. Add to the fact that Guwon is practically a living hulk. In that state of being bewildered, I probably subconsciously follow whatever Guwon is telling me to do.
Not mad at you, just excited to dive deeper into the psychology behind this scene. ヾ(❀╹◡╹)ﾉ~
I knew this story would be crazy but damn this man just watched this monster kill his wife and children the people that mattered most to him and willingly went with the guy that killed them just so he could stay alive? Cowardly