Showing Lexaeus has a sort of loyalty to Zexion (since he is titled "Silent Hero/Taciturn Stalwart"), in that he only fights Riku so that Zexion no longer has to be concerned about him, and so that they could focus their efforts against the traitors, after Vexen was eliminated. Zexion only barely concerns with them because he can't ignore Riku potentially becoming DiZ's puppet, so Lexaeus takes it upon himself to try to remove the "distraction".