He couldn't take it anymore,, drugging the concubine and making the emperor, (who killed his son mercilessly) happy
It was out of spite and for the sake of revenge. He was the only doctor who could concoct the medicine that "cured" MC's depression. So when the doctor committed suicide, he actually took the ml's hope of mc getting better and accepting him as well as making mc more miserable by taking away the only medicine/treatment that was keeping him comparatively sane.
This one is right.
The doctor's son was also a doctor who'd tried to cure Seoyo's depression, but he failed so Nareu-ye killed him (as he did with countless others). To get revenge for his son's death but also feeling angry and spiteful for all the others who have been carelessly murdered, the doctor "cured" Seoyo while simultaneously getting him addicted to the treatment, only kill himself at the pinnacle of "recovery."
He was spiteful that ML could kill so many without a second thought for something beyond their control, and mad at Seoyo for being the perceived issue to begin with. Seoyo was helped just enough to feel dependent on the drug making him feel better, and Nareu-ye was deluded enough to believe his drugged state was an improvement, so that's why the doctor said something along the lines of destroying their happiness right at the best moment. T'WAS ALL A RUSE!
Why did the doctor kill himself??