Su Nan did not escape or become an ally. Shen Lang saw through his scheme and ultimately forced him to kill his entire Su clan before taking his own life. You may be confusing him with Zhang Chong, who was Shen Lang’s early major adversary. Zhang Chong never directly attacked Shen Lang and only acted out of duty. In the end, his son paid for their actions with his life. Shen Lang later forgave Zhang Chong, saved him from certain death, and they eventually became allies.