Ultron is a rogue A.I. obsessed with nothing less than the total extermination of not just the Avengers, but all of humanity. After building the android Vision, Ultron first has him attack the envoy of the President of the United States, nearly killing both the President and his agents, then organizes the attempted mass shooting of dozens of innocent people, children included. In another instance, after Vision has gained sapience and betrayed his evil ways, Ultron holds the comatose Wonder Man hostage to try to force the Avengers to destroy Vision's capacity for emotion and turn him back into his slave, after which Ultron tries to kill them all. In his crowing moment of villainy, after failing to bombard the Earth with nuclear missiles, Ultron attempts to spread a technovirus across the planet, then watch as every machine on Earth comes alive and wipes out every human they see, paving the way for Ultron's mechanical tyranny to begin.
Cornelius van Lunt, better known as Taurus, the merciless leader of Zodiac, first establishes his ruthlessness by ordering the Swordsman cut apart into twelve pieces for his failure. A monstrous mastermind with a plan that involves splitting the Earth itself apart for the sake of power, through the course of the series Taurus regularly endangers millions of lives, from bombarding Earth with falling satellites and nuclear missiles, to having entire villages and towns wiped off the map by natural disasters, all of which he sees as mere collateral to his Evil Plan.
"Kang": Kang himself, the titular Conqueror, is accidentally freed from his temporal prison after having conquered the 31st century. Having destroyed the Avengers of his time, Kang's takeover of Earth killed millions and reduced billions more to Kang's slaves. Intending to resume his takeover of all space-time, starting backward from the 31st century to conquer epochs and waste countless more lives, Kang even ages all of New York to the point of near-oblivion just as a display of his power.
Ho Yay: The huge bromance between Hawkeye and Wonder Man, with Hawkeye calling Wonder Man "the only real friend [he'd] ever had." Not to mention the fact that it's a flashback to that scene that snaps Vision!Wonder Man out of Ultron's programming, rather than anything relating to his love interest Scarlet Witch.
Complete Monster: Issue #2—"Hail and Farewell": Baron Wolfgang von Strucker, the withered old monster running Hydra, plans to eradicate all humanity save for those in Hydra to make way for his ideal race of "homo hydra" in its place. Strucker is a megalomaniacal Social Darwinist who applies his insane standards of perfection even on his own men, killing a defector with his Satan Claw and deliberately inviting the Avengers into his base to weed out any Hydra mooks too weak to make it in his new world.
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