Thevar Magan’s screenplay is perfect, one of the greatest screenplays in mainstream commercial cinema (though Hey Ram, Virumandi and Apoorva Sahodarangal are all fantastically written). Kamal fixed most of the problems in Nayagan with his writing here. He was never satisfied with Nayagan (he is always talks cryptically about that film) , which is why he took another swing at the Godfather adaptation.
Btw, the film was originally titled “Nammavar,” but after Sivaji Ganesan agreed to play periya thevar, Kamal changed the title to Thevar Magan as a mark of respect to the legend. The title design in the film’s posters (and I think in the film also) had sivaji and Kamal’s hand drawn faces next to Thevar and Magan respectively. And yes, Thevar Magan is the perfect title for the film. It’s obvious from his character arc, the adherence to the Thevar values, both in his valor and his sacrifice. Also, there are two instances which becomes major turning points in the film in which Kamal is explicitly addressed as Thevar Magan – one by Revathi’s father after her wedding is disrupted; that’s when he decides to marry her; and then by Nazar while challenging him for the climactic duel, in which he ends up killing him.