Near the end of Joint Security Area, Sung-shik first shoots the 3rd North Korean soldier (Choi) because he is reaching for his radio.
Sung-shik then shoots Woo-jin (it is later revealed that the other South Korean soldier shot Woo-jin). Sung-shik proceeds to shoot Woo-jin 8 more times and has a slight smirk/smile on his face. He even briefly aims the gun at his fellow South Korean comrade.
Why is this?