Warning! This article contains spoilers for Guardians of the Galaxy #17
While Galactus may be seen as purely destructive without any redeeming factors, he serves a cosmically necessary purpose, and Thor's decision to put an end to Galactus' apathetic reign of terror is the Thunder God's biggest failure as a king. Galactus has long been established as a world-ending threat since his introduction in Fantastic Four #48, and though his power is immense and potentially catastrophic he has also proven time and again that he is not a villain. Galactus is as much a necessary force of the universe as he is a developed character. The Devourer of Worlds was cursed with destructive purpose after he was present for the birth of the Marvel universe. Since then, Galactus has consumed countless worlds and uses the life-giving power of planets to keep order in the universe and fend off legitimately malicious threats such as the spirit of destruction, Abraxas.
In Thor #6 by Donny Cates with art by Nic Klein and Matt Wilson, Thor is granted the power cosmic by Galactus himself to stop a universe-ending threat, the Black Winter. Before the duo are successful, King Thor uses the power boost to kill Galactus once and for all. Thor then uses the power stored within Galactus’ corpse as a sort of bomb and obliterates the Black Winter. By all counts, Thor achieved a great victory by taking out two cosmic threats in one battle, but recently it has become clear that killing Galactus was a huge mistake.
In Guardians of the Galaxy #17 by Al Ewing with art by Juan Frigeri and Federico Blee, Dormammu has crossed over into the physical world by possessing Ego the Living Planet, seemingly killing the Elder and morphing his corpse into a new monstrous form. The Guardians are fighting against Dormammu’s horde of demonic minions before a dark ritual can be completed that allows Dormammu to take control of the universe. A cosmic hero fighting alongside the Guardians, Quasar, makes it clear that if Galactus hadn’t been murdered by Thor he would have made a strong ally, for more than one reason.
Galactus and Ego were in a special group together known as the Eternity Watch, and being partners, Galactus would give Ego a power boost, one that gave the Living Planet a giant body to match his planet-sized head. This power seemingly remained inside Ego when Dormammu took him over, and the demon tapped into it giving himself an even more ferocious form. If Galactus was alive to fix this cosmic imbalance, he would be able to remove the power from Dormammu’s grasp and vanquish the villain before he could become a serious threat.
As proof of Galactus’ superiority over Dormammu, the Devourer of Worlds once defeated the demonic villain in Dormammu’s own universe. Galactus consumed a world within the Dark Dimension which gave him a magical power enhancement, one so great that it offered him the power to destroy Dormammu. Since Dormammu grows weaker in the physical world unless constantly pulling from his own dimension, he would be no trouble for Galactus. Add that Dormammu is using power that originated from Galactus himself, and he wouldn’t stand a chance against the world-ender. However, thanks to Thor's reckless decision, Galactus isn’t around to correct Dormammu’s wrongs, a failure that may mean the end of the universe.