@Phelix To me it seemed like the MC was disappointed, not angry. She did what they came there to do and MC learned the most vital parts to start piecing things together. Also his goal isn't specifically going back to earth, it's "living a peaceful life", I'd wager that curiosity is the bigger driving force.
Though her actual reason for killing him was kind of dumb.
The author is leaning in on the whole concept of "only certain personality types can think in this way" and how the MC has ambiguous morals easily influenced by his surroundings. He's not entirely wrong in regards to the personalities thing, but it's painted in such a simplistic, black and white manner.
We'll probably find out that he was actually "revived" into this world, rather than summoned, then something something evil demon lords, and oh no MC you were the demons all this time.