WARNING: The following contains major spoilers for The Death of Doctor Strange: Avengers #1, on sale now from Marvel Comics.
The shocking death of Doctor Strange has unleashed a barrage of mystical mayhem upon the Marvel Universe, with the Sorcerer Supreme no longer available to keep the supernatural threats at bay that fell under his guard. In addition to all sorts of monsters and threats emerging to menace the world, Stephen Strange is revealed to have held a whole host of secrets about his duties and responsibilities from the rest of the superhero community, including his friends on the Avengers. As the true nature of Strange's role begins to surface, Thor personally holds the late doctor responsible for the recent crossover event War of the Realms.
One of Strange's biggest responsibilities that he did not divulge to his friends and allies was that he constantly maintained a magical barrier around Earth to protect it from extra-dimensional threats. With Strange now dead, the barrier has fallen with him, allowing the Earth to be beset from all sides by invaders from all sorts of alternate universes and pocket dimensions. In a rare quiet moment between monstrous fights, with the Avengers getting the chance to regroup and lick their wounds at Iron Man's home in New York City, Thor is visibly resentful about the secret of the magical barrier in The Death of Doctor Strange: Avengers #1 (by Alex Paknadel, Ryan Bodenheim, Rachelle Rosenberg and VC's Cory Petit).
As Captains America and Marvel contemplate the cataclysmic recent events that have befallen the world, Steve Rogers reveals that Thor is frustrated and withdrawn because he believes Strange's magical barrier could have been key in not only helping protect Earth but also the invasion of the Ten Realms during War of the Realms. The 2019 crossover event by Jason Aaron and Russell Dauterman saw the evil Dark Elf Malekith form an army from across the realms to overthrow Asgard and conquer Midgard, with the sheer scale and ferocity of the attack catching the Asgardians and Earth's heroes completely off-guard.
To a point, Thor might be right in resenting Doctor Strange for not preventing Malekith and his armies from breaching various realms, including the final showdown on Earth. Strange's entire job with the barrier was to ensure that such invasions and incursions from across different worlds and dimensions would not be possible and, at the very least, could have provided some sort of warning to the Avengers when Malekith led his forces to Earth. On the other hand, what made Malekith's rise to power so effective in the first place was the amount of subterfuge and manipulation he employed, with Odin even ambushed in the heart of Asgard by Dark Elf assassins because of how Malekith was able to travel between realms undetected.
War of the Realms completely brought Asgard to its knees and was an incident so harrowing that the memories of it are still fresh in the mind of everyone on Asgard. Casualties from Malekith's assault, including the massacre of the Valkyries, ran particularly high over the course of the story and made Thor begin his subsequent reign as King of Asgard on something of a tragic note.
While Strange isn't around to be admonished by Thor for failing to intervene in time, the God of Thunder definitely has some unresolved issues to work out now that he knows how much more the Sorcerer Supreme could have done.