Warning: SPOILERS for Avengers #65
The hammer of Thor can only be lifted by those who are worthy - but sometimes, a person can be so UN-worthy that the hammer will do more than fail to move...it will outright kill the would-be wielder. Mjolnir is one of the most iconic weapons in the Marvel Universe, so much so that Thor is rarely seen without it (moments when the hammer is stolen or destroyed being the obvious exceptions). In Avengers #65, Mjolnir finally judges someone so unworthy that it decides the only course of action is to fly them straight into the sun - and that person is Thor himself.
2022 was a tumultuous year for the God of Thunder. For the first time in the character's history, Thor felt Mjolnir getting heavier and heavier, and occasionally he failed to lift it at all - but other characters like Captain America and even Loki could hold it with ease. Eventually, Marvel revealed the hammer was possessed by Magog, and the ensuing fight ended with the destruction of Mjolnir and the death of Odin. The Allfather's spirit lives on inside the reconstructed hammer, however, guiding Thor on his continuing adventures as King of Asgard.
But in Avengers #65, written by Jason Aaron & Javier Garron with art by David Curiel, the hammer judges Thor unworthy long before he became an Avenger. A variant Loki who now goes by the moniker Avenger Prime remembers one of the happiest days of his life: the day when Thor died. "That mallet with a mind of its own...had flown the young prince of Asgard into the sun." This change in trajectory kills Thor and also alters the trajectory of Loki's life forever.
Thor's Hammer Judged Him Unworthy - And Flew Him Into The Sun
Thor's hammer has a sentience of sorts, but this is the first time that the hammer has outright killed Thor by refusing to listen to his demands. Considering the fact that many villains have attempted to lift Mjolnir over the centuries, this means that Mjolnir judged Thor more unworthy than even Loki. Why Thor didn't let go of the hammer is anyone's guess; either the stubborn prince thought he could control the hammer to his last, or Mjolnir has the ability to bind one's hands to the hilt (a power never before demonstrated).
It must be noted that this is a variant of Thor and not the "true" Thor of the 616 reality. What exactly this Thor did to warrant such a judgment from his own hammer is unknown and most likely will never be revealed. For his part, the Thor of the 616 universe still has his reforged hammer - hopefully without the weapon's homicidal tendencies.