The way I interpreted the Liege Maximo (before The Fallen was made), was that he was a 'backup plan.' Primus and Unicron represented the balance of good and evil, chaos and order, life and death. But, if Primus was to actually succeed in destroying Unicron (and in the comic continuity, he did, as the Singularity concept hadn't been made yet), there would have to be a new source of balance, and the universe put it in Primus' own children.

On the other hand, The Fallen defecting to Unicron was a result of his own duties corrupting him, not part of the universe's own plans. If the Liege allied with Unicron, it would defeat the purpose of having a "backup evil."

(in my fanon, I like to also see Devil Z, Violen Jiger, and Dark Nova as other "backups")

 

Well before the Fallen was a gleam in Pat Lee's eye I interpreted the Liege Maximo as the opposite mystical bot of the Last Autobot. Not to mention both heralded the end of the Transformers Marvel run. Mind you both were creations of Simon Furman.