In the Marvel Cinematic Universe, Thanos uses the Infinity Stones to erase half of all life in the universe, believing it is a necessary sacrifice to restore cosmic balance in an overpopulated world. It’s quite a different goal than his comic book counterpart, whose intentions are much more self-serving; he destroys half of life to please the living embodiment of Death. Galactus’ existence is crucial in the continuation of the universe — with his hunger playing a pivotal role in balancing life and death. However, unlike Thanos, he destroys planets not for pleasure but to fuel his energy.
Before becoming Galactus, he was a human-like being named Galan from the planet Taa. With the destruction of the Sixth Cosmos, the Sentience of the Cosmos approached him to become reborn in the new universe. As a result, Galan became Galactus, feasting on planets in the new reality to prevent his death and potentially causing a new Big Bang and destroying the Seventh Cosmos. Additionally, it’s explained Galactus’ existence is meant to test worlds to see whether they are worthy of continuing.
Without Galactus, the universe might be thrown into peril. He takes lives to ensure the cosmic scales remain level. Thanos has a similar mindset in snapping half of all life in the Marvel Cinematic Universe, albeit his machinations are entirely self-driven by his beliefs.
