After sacrificing his role as the God of Thunder to save Asgard and serve as Galactus' herald, Thor allowed his master to destroy his home after learning what happened in his absence. In What If... Thor #1 by Marvel Comics, Thor returned to Asgard from serving the Devourer of Worlds after he was informed of the death of Odin. But, upon seeing Loki take credit for his father's death and rule in his place, Thor lets Galactus destroy his home.
Thor has a tumultuous relationship with the Devourer of Worlds, serving as his herald both in continuity and out of canon. In Donny Cates and Nic Klein's ongoing Thor run, the God of Thunder became Galactus' herald after learning about the threat of the Black Winter and how the cosmic force could destroy Asgard. However, his time with Galactus ended in a surprising fashion with the cosmic juggernaut's death, as the planet-eating villain's helmet became a permanent part of Asgard. But, before that, Thor first became Galactus' herald in a non-canon What If...? story, where he willingly let his master eat Asgard after learning the truth about Odin.
In What If #1...? What If Thor was the Herald of Galactus? by Robert Kirkman, Michael Avon Oeming, Val Staples, and Artmonkeys' Dave Lanphear, Galactus attacks Asgard, with Thor killing his unnamed herald in the subsequent action. However, after seeing the destruction the massive being caused, the God of Thunder agrees to become Galactus' new herald in exchange for leaving his home alone - a similar promise Silver Surfer made to protect Taa. Thor transforms into his herald, but after serving the Devourer of the Worlds and directing him to less populated and more dangerous planets to feed on, he gets a message that Odin has been slain. Upon returning to Asgard, Thor learns Loki killed their father and rules. In response, Thor allows his closest allies to escape and tells Galactus he can destroy his former home, as there's nothing left for him.
Thor turning on Asgard is one of the worst things he's ever done - even if the story is out of continuity. But for the God of Thunder, learning of Odin's death and seeing his former home a shell of what it once was broke him, leading him to tell Galactus Asgard is his to destroy. Under normal circumstances, Thor wouldn't make the same decision, but with Odin dead and most of Asgard in ruins, the circumstance proves to be too much for him. In Thor's eyes, there was nothing left for him on Asgard.
The What If...? story shows Thor at his most defeated. Deciding to work alongside Galactus to save his home was already an incredible sacrifice. However, after learning about Odin's death and Loki's rule, while seeing what Asgard became when he was gone, Thor OKs Galactus destroying his home, showing how broken he had truly become.