To win her over, Thanos becomes hell-bent on sacrificing as many lives as possible in her name. You might not remember Death from Avengers: Infinity War because the Russo Bros. didn’t write her in, and instead focused on portraying Thanos in the reluctant, ends-justifies-the-means savior role.
Through a series of events, Death rejects Thanos, which literally means that he cannot die. Like, ever. So no one is killing Thanos. But that doesn’t mean he can’t be beaten.
Who can defeat Thanos?
You may have recalled that, in one of the post-credit scenes at the end of Infinity War, there was a slow, zoom-in crawl on a Golden sarcophagus of sorts. In that beautifully-crafted iron maiden is none other than Adam Warlock, an artificially created, “perfect” human being. In the comics, Warlock battles Thanos and initially loses.