I know this sounds like a crack AU, but hear me out-
Starscream approaches the Autobots one day under the guise of 'leaving the Decepticons to join the Autobots'. ...None of the Autobots believe him, but Optimus convinces them to give Starscream a chance, under the idea that anyone can change if only given the opportunity.
To avoid as much potential sabotage as possible, though, Optimus tasks Starscream with the most benign thing he can think of: helping the humans at some zoo, and promising Starscream that if he proves trustworthy, he'll let him do more.
Starscream is upset, but not wanting to blow his cover, he agrees; though not without complaining the whole time about having to be around "such disgusting organic creatures".
Starscream does the bare minimum of effort, though forcing himself to be gentle with the animals so he doesn't hurt them and blow his cover. Until he meets 'Mr Hiss', the honey badger.
Starscream finds Mr Hiss after the tiny creature tried to attack him one day when he escaped his enclosure. Starscream is confused, seeing as most creatures of this size usually run away from him. He learns later from stories the zookeepers share that 'Mr Hiss' is infamous for escaping his enclosure and making trouble.
Starscream is intrigued, but in a detached way, like one would be intrigued about an ant. He returns to the zoo one day, though, only to find 'Mr Hiss' got into the lion enclosure and was beaten so badly that he almost died. Starscream is put in charge of taking care of the badger, and he does everything possible to help the badger heal.
...Not because he cares about Mr Hiss, obviously, but just to make sure the Autobots think he does.
Starscream's help pays off, but as soon as Starscream hears Mr Hiss escaped again, he finds Mr Hiss at the lion exhibit, trying to break in for revenge.
Starscream picks up the little troublemaker and chastises him as he walks him back to his enclosure. "Foolish organic! You already nearly died to those creatures! Why risk so much just to get revenge-..." Starscream pauses, and he has a moment of self-reflection.
After that, Starscream visits Mr Hiss's enclosure more often, and slowly begins visiting the others, too. Starscream actually helps with the animals. Soon, Starscream realizes he's getting attached. He tries to deny it, but he can't bring himself to say he hates these creatures.
Starscream reports back to the Decepticons less and less. Eventually, Starscream refers to himself as an 'Autobot' without hesitation.
Oh, sure, Starscream is still a bit... 'rough around the edges', but he doesn't want to hurt Earth or its creatures anymore... so that's an improvement, at least?