Why is Tokchae a prisoner? What reasons does he give for not leaving his father?

The first sentence of the story gives readers important setting details:

The northern village at the border of the Thirty-eighth Parallel was snugly settled under the high, bright autumn sky.

The 38th Parallel is an important location because it means that the story is taking place at the dividing line between North Korea and South Korea. During the war, villages located on or near this line changed hands several times as each side pushed their front line deeper into enemy territory. Tok-chae and Song-sam grew up as boyhood friends in this particular village, but the war caused them to end up on opposite sides of the conflict. Tok-chae has been captured because he's the "enemy," and Song-sam is the officer in charge of taking his childhood friend to the prisoner camp.
As Song-sam is escorting his friend, Song-sam has a flash of anger because Tok-chae wound up fighting for the other side:

Song-sam felt a sudden surge of anger in spite of himself and shouted, “So how many have you killed?”

Song-sam then asks why Tok-chae stayed behind to fight instead of fleeing like he did. Tok-chae says that he wanted to, but his father wouldn't leave. His father saw no point in running away and didn't know what he would do in another place. Tok-chae simply couldn't bring himself to leave his father. Tok-chae says,

I wanted to be with him in his last moments so that I could close his eyes with my own hand.