The Dread Lords differed from the Arnor in that they perceived the passage of time. The Arnor were a wasted effort from the perspective of the Dread Lords.  Power without purpose is evil in their eyes and the Arnor never had any purpose other than to exist.

A popular story told to an Iconian long ago was representative of how the Arnor and the Dread Lords viewed the meaning of life. When an Arnor first saw a glacier, he sat and watched its ebb and flow over centuries -- doing nothing else. Because to an Arnor, time meant nothing. They don't eat food. They don't need to sleep. On the galactic scale, seeing such massive change to an environment that glaciers could bring was endlessly fascinating.  The same could have been said of plate movement (except that even the Arnor can't survive in the vacuum of space indefinitely and thus could not watch continental plates move easily).

To the Dread Lords, such is a waste. The Dread Lords had endless purposes to achieve in the eternity available to them. It was, for instance, the Dread Lords, not the Arnor, who apparently came to understand that existence was not infinite. That when the Telenanth was unleashed, it altered the laws of physics in the universe causing space to accelerate its expansion which the Dread Lords conclude will mean that eventually matter in the universe will disipate and that even the Dread Lords would pass out of existence. With such understanding, the Dread Lords strove to accomplish things ranging from traveling to other dimensions and realities, mastering the current universe. Seeing if other universes existed and whether they could travel to them.  Whether they succeeded is unknown other than the fact that they did manage to discover a pocket universe which, ironically, they were ultimately trapped within.