Only those who are worthy may lift Mjolnir, the mighty hammer of Thor, God of Thunder. However, worth has little to do with actual morality. For every Captain America or Jane Foster who can lift the hammer thanks to their good, moral souls, there are a multitude of wicked individuals who -- for whatever reason -- are worthy of lifting Mjolnir.
These villains might not be noble or even all that good, but they possess qualities that make them worthy to lift Thor's hammer. From fellow Gods to literal Nazis, when these individuals lift the hammer up, it begs all sorts of questions pertaining to Mjolnir's standards for worthiness -- or, at least, how villains have found ways to subvert Mjolnir's capabilities to make themselves worthy.
Loki
Loki is famously one of Thor's greatest adversaries. Thor has repeatedly used Mjolnir to pin Loki down, across various continuities. However, in Thor #4, Loki lifts the hammer -- though the reasons for this have yet to be revealed. There are, however, a few potential ideas.
At the end of War of the Realms, Loki assumed the throne of the Frost Giants and became their king. Perhaps the honor of leadership has resulted in Loki becoming worthy of Mjolnir. In recent years, he has also tried to become a hero. However, it's also possible Loki benefits from good circumstance. The hammer has, as of late, been harder for Thor to lift. Perhaps Thor's ability to lift the hammer has been transferred to Loki. Due to Loki's uncertainty when approaching the hammer, it's safe to assume Loki is unaware of an enchantment which may allow him to lift the hammer. Regardless, the reason behind his new ability should be revealed in coming months.
Hydra Captain America
Steve Rogers is worthy of lifting Mjolnir. This much became obvious during the Fear Itself event. However, in Secret Empire, it turns out Steve Rogers retains his worthiness, regardless of whether he's actually good. Therefore, it leaves the world in shock when an alternate version of Captain America -- one who's loyal to Hydra -- finds a way to lift Mjolnir. Many fall in line behind this evil version of Captain America, seeing the hammer as proof that Cap is still the same hero, even if he follows the enemy.
However, it's later revealed Hydra found a way to deceive the world. Unknown to this alternate version of Steve Rogers, Elisa Sinclair (Madame Hydra) used the Cosmic Cube to re-write the laws of Mjolnir to allow Cap to lift the hammer. This temporary re-write of reality remains unknown to Steve, which leads to a critical mistake when Hydra Cap and the true Cap come into conflict with one another and Hydra Captain tries to lift the hammer -- only to fail. In his horror at the whole scenario, the true Steve Rogers lifts the hammer and uses Mjolnir to remind the world why he is deemed worthy.
Red Hulk
To lift a hammer up, a hammer needs to be heavy. For an object to have mass, there needs to be gravity. This of course begs the question: what happens if Mjolnir is floating out in space without gravity to pull it down? Can someone -- even if they aren't Thor -- lift it without being worthy of the hammer?
The answer to this question is uncovered when Red Hulk fights Thor in space. With the hammer floating weightless above the Earth, Red Hulk doesn't so much lift the hammer as become the biggest source of gravity present to control where the hammer comes to rest. It just so happens Red Hulk makes sure the hammer rests flat against Thor's skull -- over and over again.
Ultimate Magneto
Arguably one of the most vile people to ever wield Mjolnir for the most wicked of intentions, Ultimate Magneto infamously uses the hammer during the Ultimatum story to augment his own magnetic ability and reverse Earth's polarity.
Unlike the 616 Magneto, the Ultimate version of the X-Men villain doesn't have a good bone in his body -- and even though the 616 Magneto has limited control over repulsing the hammer with magnetism, he can't truly control it. Ultimate Magneto is pure evil, seeking only to inflict pain and agony upon human and mutant alike. However, this Magneto doesn't truly lift the hammer. Rather, he uses his magnetic ability to manipulate the Uru -- metal forged from a collapsed star -- that comprises the hammer.
Doom
Doctor Doom might appear to be the last person worthy of wielding Mjolnir. Nevertheless, there is one moment in Doom's life where he's deemed worthy to lift Mjolnir. During Ragnorak, Thor is separated from Mjolnir. Masterless, the hammer plummets to Hell, where Doom is imprisoned during the lead-up to Civil War. Since heroes are distracted everywhere, Doom seizes his opportunity and uses the hammer to battle his way out of Hell. Once Doom escapes, he once again becomes unworthy of the hammer.
Oddly enough, however, one individual actually worthy of wielding Mjolnir during this time is Ragnorak himself, the Thor clone who murders Goliath for Tony Stark, his creator. It seems peculiar that Mjolnir falls into the hands of two wicked villains in a row during Ragnorak. Perhaps Thor's demise left Mjolnir confused.