This kind of reminds me of the old analogy about the fox and the rabbit. When the fox chases the rabbit, the rabbit will usually out run the fox because the fox is running for his dinner, whereas the rabbit is running for his life!
During the Korean War (1950 - 53), the South Koreans were fighting for their lives and their country. They were pinned down in the bottom of a peninsula with all of the forces in the north coming down on them. The war didn't officially end in '53, but was halted with a cease-fire "truce agreement."
I would surmise that the ferocity of the ROK in Vietnam (1959 - '75) might have been a carry over from the mentality developed a decade earlier - - fight to win, or lose everything! The unrestrained tactics of the ROK might well have been a continued fight against the communist forces that had threatened, and still do threaten South Korea.
However, I do believe that quite a bit of the ROK's fierce reputation came from their hand-to-hand combat and ability to kill the enemy at close quarters on the battlefield. Many of the stories that I have read about the ROK related more to that than just indiscriminate shooting of villages, although I'm sure both would have contributed.
CM D.J. Eisenhart