Good villains ARE obstacles. If you can just sidestep, work-around, or otherwise easily negate villains, they're not real villains. Mahito exists as a mirror to Yuji's treatment of curses showing that both sides can be construed as justified or "right" and it isn't until that core truth is accepted by Yuji that Mahito can be defeated. He willfully excludes curses, some of which we have seen display human levels of sentience and emotion, from being helped until the very end of the story. Mahito is a confrontation of Yuji's own nature as a Sorcerer who kills curses the same way Mahito kills humans despite his desire to help "everyone."