In 1965, both countries had the same GDP per capita, however South Korea nowadays has a GDP per capita 5 times greater than Thailand?

Who was responsible for this?

I would guess Thanom Kittikachorn because his rule was marked with corruption. By the end of his rule, Thailand’s GDP per capita was $300 while South Korea’s was $500 which isn’t such a large margin but the political instability that came after his exile was so great that it damaged investment opportunities.