Galactus of the Marvel Universe is an exciting character to me for a number of reasons, although most of the excitement comes from his having “limitless power” ala the Power Cosmic and even more so, the dubious conception of him as an amoral being. These beings (and some who claim to be but aren’t) tend to shake up the world around them simply through their presence, and the titanic Galactus is surely no exception. But is he actually amoral, or just a pretender?
Let’s begin on common ground ala the dictionary and Wikipedia again:
Amoral: being neither moral nor immoral; specifically : lying outside the sphere to which moral judgments apply 
How can we judge a being as being neither moral nor immoral? What lies “outside the sphere to which moral judgments apply”? One thing that fulfills this definition is necessity, or rather, that which is necessary. In terms of humanity, this more directly correlates to survival and a specific subset of it.
What is necessary for a human to survive?
— Reproduction (for the species to survive)
— Shelter (to a lesser extent, depending on environments)
There is an important but very subtle point that is implied here, and that is that the intent matters. For example, it is immoral to kill a person out of spite or envy, yet it is neither moral nor immoral to kill a person to cannibalize them to save oneself, although this does hurt the species on a small scale.
This has been a not-so-subtle attempt to draw a metaphor between the previous situation and Galactus’ “plight”. He must feed on planets at regular intervals or die, similar to humanity on a larger scale. Galactus, for the most part, has no preference between planets with sentient life and those without, the exception being a brief “addiction” he encounters back in Galactus: The Devourer. He becomes obsessed with consuming sentient life to the point where he is no longer able to act rationally. He can therefore be considered to be insane and thus not accountable for his actions during this brief spell. Furthermore, he still has no conception of right or wrong while during this time, and is only compelled to feed for the sake of feeding.
Some people argue that any being who kills another for sustenance is immoral, as they could let it live and eat plant life instead. But plant LIFE is still LIFE, and those who argue that the destruction of all life is immoral must answer this as well.
Relative to himself, Galactus is also amoral for the reason cited two paragraphs ago: although he knows why you and I might believe that devouring planets is immoral, he simply does not believe that this action is moral or immoral. Devouring planets for sustenance is simply “neutral”, to put it bluntly.
Yet it is not just that he sees this as neutral as much as he sees everything as neutral in the grand scheme of things. He is intelligent enough to realize what others believe is good and evil, yet he is not bound to their value systems and leads a simpler existence. Galactus is amoral, and to him, he just is.