Later writers, in particular comic scribe Simon Furman, would flesh out Unicron where the film failed. Building on the character’s devilish looks (as designed by Floro Dery), Furman established Unicron as the Cybertronian Satan. He wishes to destroy Cybertron because the planet is actually the dormant, transformed body of Primus, his twin brother and the Transformers’ creator. The planet contains Primus’ soul, so that’s why Unicron is vulnerable to it.
None of this grand fantasy is present in the film as it stands, though. All the gravity of Unicron (beyond the literal) comes from Welles. Miraculously, his dispassionate delivery actually works: Unicron sounds like a bored god, indignant that he must lower himself to communicate with lesser beings. Whenever the villain does get moments of personality, Welles’ famous wit shines through (e.g. “Nobody summons Megatron!” “Then it pleases me to be the first”).
Welles undoubtedly deserved a better farewell role, but even phoning it in, his talent shined through.
