WARNING: The following contains SPOILERS for Resident Alien season 1, episode 10, "Heroes of Patience."
In Resident Alien’s season 1 finale, the main question viewers are left with is: why did Harry change his mind about his mission to destroy Earth? Resident Alien follows the experience of an alien being posing as human doctor Harry Vanderspeigle; after crash-landing on Earth, the alien finds himself reluctantly roped into a murder investigation and becoming the local doctor in small-town Patience, Colorado — and forming friendships along the way.
Despite his sole mission being to eradicate mankind in Resident Alien, Harry takes to being human when he's forced to interact with the residents — especially after connecting with Asta Twelvetrees, played by Sara Tomko, who feels as alien on Earth as Harry actually is. Played by Alan Tudyk, the alien experiences an array of common human experiences, including, anger, annoyance, romance, drunken dancing, and hangovers. Harry forms relationships with various people — but despite his conflicted feelings, he continued his mission.
Yet, Harry abandoned his plans at the last minute in the Resident Alien season 1 finale. His desire to be Asta's friend was the catalyst for Harry's change of mind and heart. Asta is one of the few humans who knows Harry's secret, and although she initially rejected him in episode 10, "Heroes of Patience," in the end, she pleads with him to change his mind — and he listens. Overall, Asta’s open nature and strength of character made an impression on Harry.
Considering the sum of all of Harry's experiences since landing on Earth — the burgeoning friendships, the heartache, and the sense of purpose being the town doctor brought — it was inevitable that Resident Alien season 1 would end with Harry changing his mind. This becomes even more clear when Harry has an imaginary conversation with the real (decaying) Dr. Vanderspeigle, in which the alien its that despite the ups and downs, he feels connected to his environment and to hope. The show repeatedly hinted that Harry was becoming human: even though Harry was adamant about destroying the planet, he hadn't anticipated that the device would harm him. In light of discovering it would, Harry was forced to alter his plans — setting up the finale's climax, in which he changes his mind.
Ultimately, his last-minute change of heart boiled down to one thing: love. His love for Asta and her friendship was instrumental in his change of plans, coupled with his own growing sense of humanity. With just over a minute to spare, Harry leaves before the device detonates and surmises it’s the choice to love that makes humans strong. Whether Harry will return to Earth with stowaway Max in Resident Alien's season 2 is still up in space, but there’s hope for Harry and Asta yet.