Warning: SPOILERS for Avengers #65.Marvel's new Loki variant, who calls himself Avenger Prime, accomplished what not even Thanos could ever do: killing the entire universe. While Thanos' quest for nihilism and the love of Lady Death brought him very close to extinguishing all life on several occasions, this Loki managed to do it by making the rest of the universe pay the price for his powerful spells.
Loki and Thanos are two iconic Marvel villains who usually have very different goals. Loki has very "mundane" desires for a god: killing and/or humiliating Thor, conquering the throne of Asgard, acquiring power, and so on. Of course, this was before Marvel's Loki transitioned from bona fide villain to lovable scoundrel anti-hero. Thanos, on the other hand, has always been more metaphysical in his approach to villainy. In his most infamous feat, Thanos erased half of life in the universe using the Infinity Stones, but he never went as far as Avenger Prime did.
Marvel's "Greatest Loki" Killed Every Living Being In His Universe
Avengers #65, by Jason Aaron, Javier Garròn, and David Curiel, gives readers the backstory of Avenger Prime, who has recently been revealed as a version of Loki. In this Loki's universe, Thor was killed by his lack of control over Mjolnir, which led to Loki's easy ascent to the throne of Asgard. Peering into the Multiverse, Loki realized that in every reality he is destined to be the reason why the Avengers get together, so he decided to prevent the rise of the Avengers (and every other hero) in his world. However, this meant that there were no heroes to stop the real, big threats to the universe, such as Galactus or Thanos himself. Loki had to fight them and won, proving that magic is indeed the strongest power in the Marvel Multiverse. But great magic comes at a great cost, and Loki had to sacrifice the lives of every single being in his universe to win.
Clearly, the Loki who would then become Avenger Prime did not care for saving the universe, only for breaking what he perceived as "the curse of Lokis": being destined to lose. This is why he prevented the rise of the heroes, and why he could not let the other villains win. Killing the entire universe is quite a feat, but readers of Avengers have already witnessed similar aftereffects of powerful magic when Doom Supreme attacked Avengers Mountain. It was revealed that each spell has a cost, but skilled and cruel magic users can deflect that to other people. This is what Loki did, and the price to pay for spells powerful enough to defeat the Celestials, Galactus, and Thanos, was the extinction of life in the universe.
Loki later repented and assumed the identity of Avenger Prime to assemble the Avengers of the entire Multiverse to fight against Mephisto and Doom Supreme, but nothing makes up for the crime of killing an entire universe. Not even Thanos has ever wanted to achieve destruction on such a universal scale (and when he did by mistake, he made sure to bring everything back to normal), which must make this Loki the biggest villain in the Marvel Multiverse (or at least the most effective).
Avengers #65 is available now from Marvel Comics.