Mjolnir can only be lifted by beings who are worthy, which I generally interpret as honorable. Just like this time, Should attempted murder make him unworthy? When Thor discovered the prophecy that he would die battling Loki's son Jormungandr, he began a quest to kill his own nephew. Is it explained how he can still wield Mjolnir?
Considering both Thor and Jormungandr were going to die in the prophecy, Thor and only Thor benefits from murdering his nephew. If he let fate play out he would kill the serpent anyway. This seems selfish. Is he still worthy after attempting to murder his own nephew when no one was in danger?
(The Mighty Thor #486)
Loki's son Jormungandr, born by Angerboda.
This question considers that no matter how evil a villain is, heroes in Marvel never seek out to kill them. (well, honorable ones who can wield Mjolnir don't). The honorable solution to evil has always been imprisoning or containing it somehow. Only an accident, the chaos of war, a force majeure, or the hands of a villian actually kill. For example, Thor has never even considered killing his own brother even after Loki endangered all of Midgard.
How is someone who is actively seeking out a sentient being to murder them, with no provocation, still worthy of Mjolnir?