Warning! Spoilers for The Death of Doctor Strange #4 by Marvel Comics
Marvel Comics has just revealed the person responsible for the death of Doctor Strange and it turns out a fellow magic user took down the Sorcerer Supreme. In The Death of Doctor Strange #4 by Marvel Comics, the time remnant of Stephen Strange figures out Kaecilius is responsible for his future self's death - and he had a very personal reason for murdering him.
The death of Doctor Strange shocked the Marvel Universe, as the Sorcerer Supreme was murdered in cold blood in the Sanctum Santorum. His death caused massive fallout, as the magical barrier Strange created protecting Earth fell, allowing dimensional threats who couldn't previously enter to arrive on the planet. The most significant danger was the Three Mothers, predators who fed off magical heroes and were after Strange's former love, Clea. In Doctor Strange's place, a time-displaced version of the hero arrived to solve his future self's murder. And now, the hero dramatically solved the case.
In The Death of Doctor Strange #4 by Jed MacKay, Lee Garbett, Antonio Fabela, and VC's Cory Petit, Doctor Strange confronts Baron Mordo at Castle Mordo in Transylvania, where the evil sorcerer convinces him he didn't kill the hero. Mordo explains someone hid Doctor Strange's cap and Eye of Agamotto in his home in an attempt to frame him. He tells Doctor Strange he wouldn't have lied about killing him because it's something he's desired for years. On a later walk with Bats, the ghost dog suggests the person responsible for killing Strange wanted to do damage to both Sorcerer Supreme and Mordo, leading the time-remnant of the hero to realize one person fits that bill: Kaecilius.
In Antarctica, in a meeting with the dread warlords, Strange deduces in an effort to avoid the Three Mothers and the Peregrine Child, the warlords ended up using Kaecilius as a pawn to take down the magical barrier surrounding Earth so they could be in a safer realm. Keaecilius had a perfect motivation, as he could take down the Sorcerer Supreme (and gain his power while taking his hands) and slay the man who Baron Mordo wanted desperately to kill. As the former servant of Mordo, Kaecilius was abandoned by his master after he was sent to the Purple Dimension. In murdering Strange before him, Kaecilius took one of the things Mordo desired more than anything else.
The issue ends with Kaecilius threatening Doctor Strange with his own hands that he will kill him again and perhaps more concerning, the arrival of the Three Mothers and the Peregrine Child. While Kaecilius has been identified as the murderer, Doctor Strange and the rest of the sorcerers have much bigger things to worry about in the present. The Death of Doctor Strange #4 is in comic book stores now.