When Martin Lee heard that Kraft wanted to head off into space on his own, his first instinct was to stop him. But just as he was about to speak, he realized he didn't seem to have any reason to object.
Earth was too small.
Martin Lee had firsthand experience with Kraft's black technology. Until now, various concerns, along with his desire for revenge, had forced Kraft to operate in secret. Now that he was finally free of those restraints, Martin felt he shouldn't become another obstacle in Kraft's path.
Although Martin Lee had misunderstood the situation, that didn't stop him from choosing to support Kraft.
It wasn't as though Kraft couldn't tell Martin Lee the truth, but to put it bluntly, what would telling him accomplish? Martin's negative energy was certainly impressive in some respects, but it would be of little use in an interstellar war.
Besides, they couldn't simply abandon all their businesses on Earth. Someone had to stay behind to manage them.
Most importantly, once Kraft mastered the quantum communication and interstellar teleportation technologies from his dreams, returning to Earth whenever he wanted wouldn't be particularly difficult.
So Kraft offered Martin Lee no further explanation. He jumped into the enormous crater, retrieved the Mind Stone, and placed it in a box.
Now that he knew Thanos was already searching for the Infinity Stones, he had to handle this thing carefully. Viewed that way, handing the Space Stone over to Asgard for safekeeping had actually been the right choice.
Still, Earth certainly lived up to its status as the main stage of the Marvel Universe. Three of the six Cosmic Gems were here, and even after sending one away, two remained. It would have been stranger if Thanos didn't come knocking.
They returned to the airport in Sokovia