The divided nature of evil

Morgoth might have been the most powerful being on Arda, but from all the battles and dark creations he forged (Carcharoth, the dragons, raising mountains, bearing the Simarils for centuries and losing his foot, etc.) his vitality and inherent power was being drained (this being illustrated in the forms Morgoth could or could not form for himself).

Sauron and the other captains of Angband were commanded by Morgoth, but he (Sauron) would eventually (in some distant future) have tried to supplant Morgoth as his strength was depleted (that is to say only if the War of Wrath never occurred).

Tolkien presents the forces of evil as continually in competition with each other, because they are essentially selfish. They co-operate only for some immediate advantage. So the orcs make a show of good fellowship, now and then, but fight each other at any time, and keep on dividing into smaller and smaller factions. Saruman will stab Sauron in the back any time, and Wormtongue will do the same for Saruman. So it is in the nature of things that Sauron would make a grab for supreme power if he thought he could get away with it.

In C.S. Lewis's The Screwtape Letters, addressed by a senior devil to a junior one, as uncle to nephew, they seem very friendly. But what the nephew wants is to betray his uncle so he can get his job, and what Screwtape actually wants is his nephew -- or at least a piece of him. That's friendship down there in the Lowerarchy, and I think Sauron and Morgoth would be the same.

(Tom Shippey)