Here are some of the main antagonists in the Avengers movies:
* Avengers (2012): Loki is the main villain, seeking to conquer Earth with an army of Chitauri.
* Avengers: Age of Ultron (2015): Ultron, a sentient artificial intelligence, plans to destroy humanity and create a new world.
* Avengers: Infinity War (2018): Thanos is the main antagonist, aiming to collect all the Infinity Stones to wipe out half of all life in the universe.
* Avengers: Endgame (2019): Thanos is again the main antagonist, having already achieved his goal of wiping out half of all life. The Avengers are now trying to reverse his actions.
So, the "bad guy" in Avengers depends on which movie you're asking about!