The Tamil telling that carries this story is the Kanda Puranam, ascribed to Kachiyappa Sivachariyar of Kanchipuram and generally dated to around the fourteenth century, which reworks Sanskrit Skanda material into Tamil verse and became the standard scriptural account for Murugan devotion in the Tamil country.
What that telling does with the ending is unusual among battle myths. Surapadman, split by the spear, takes the form of a mango tree, and the two halves of the tree become the peacock that Murugan rides and the cock on his banner. The enemy is not annihilated and is not sent to a hell. He is placed permanently beneath the god and in front of him, in the two emblems that identify Murugan wherever he is depicted.
Tamil commentary reads this as the asura's own request being granted. Having fought the god face to face, he asks to remain in his presence, and the form that presence takes is service. The theology is close to what Vaishnava traditions say of enemies who are liberated by the intensity of their opposition, but the Murugan version is more concrete, because the result is visible on every image and every temple flag.
It is also why the festival ends in celebration and in a wedding rather than in triumph over a defeated enemy. Devotees at Soorasamharam are not watching a villain destroyed; they are watching a hostile power brought inside the god's household, which is what the tradition holds happens to whatever is set against him.