Warning! Spoilers for Eternals #12 by Marvel Comics
The Eternals just gave the Avengers a reason not to trust them, gifting the superteam the perfect excuse to hate the ancient race of heroes. In Eternals #12, Sersi reveals to the Avengers that Thanos not only returned but had become the Prime Eternal and was defeated without them knowing. When Iron Man questions how they couldn't tell them Thanos returned to their planet, Sersi says the Eternals are landlords to humans and admits they shouldn't be trusted.
The Avengers and Eternals have always had somewhat of a rocky relationship. After all, when Earth's Mightiest Heroes found out there was an ancient group of heroes who could have intervened and helped them in some of their greatest battles but chose not to, it understandably grew a wedge between them. On the flip side, the Avengers, including Iron Man, hated that the Eternals would act in cosmic fights the team had no control over or knew little about. While the Avengers and Eternals have fought side-by-side before, often, the Eternals act in their own interests and not on behalf of humanity. The most glaring example came from the MCU when the Eternals didn't even intervene when Thanos arrived on Earth, as it wasn't part of their mission.
In Eternals #12 by Kieron Gillen, Esad Ribic, Matthew Wilson, and VC's Clayton Cowles, Sersi finally reveals the truth about Thanos' return, the Mad Titan becoming the Prime Eternal, and the Eternals themselves defeating the supervillain. Iron Man is shocked that the Eternals would tell them about Thanos, telling Sersi, "it's our planet." Those words hit a nerve for the Eternal. Sersi tells him how can she not patronize him as he's only got a "few decades of life left" when her race will live for thousands of years. She says Earth is "our planet" and will always be, calling the Eternals a "good landlord" to humanity. Sersi ends the conversation by admitting the Avengers shouldn't trust the Eternals considering all they've endured.
The Eternals have lived on Earth for over a million years, and in the grand scheme of things, the current Avengers team is just a tiny blip on the overall time they've spent on the planet. Sersi's words might be strong, but she's not off-base with her comments, even if they are patronizing. Still, the Avengers have to hate that Thanos returned without their knowledge or input, showing once again, why distrust has built around the superteam and the Eternals. The biggest villain in the universe was roaming free, and they had no idea. Keeping secrets that big often have catastrophic consequences. Thankfully, in this case, the Eternals dealt with Thanos before it was too late - even if they nearly lost.
The Avengers will be a part of the upcoming war between the Eternals and X-Men and considering the events that just unfolded, they have every reason to join the mutants in the fight. Sersi also admitting they can't be trusted doesn't help their cause. The Eternals' secrets, including the most recent revelations about Thanos, have given the Avengers more than enough ammo needed to truly hate them.