Why did Cherry become a spy for the greasers in The Outsiders?
She wanted the fighting to stop because she hated fighting; secondly, she had a huge crush on Dally, so she would do whatever it took to help the greasers; and finally, after meeting Ponyboy she realized that not all greasers are the same.
You have already provided three good reasons for Cherry's decision to spy on the Socs for the greasers. She does abhor the violence that exists between the greasers and Socs; she is attracted to the dangerous side of Dally in much the same way that she was attracted to Bob; and she genuinely likes Ponyboy. I think above all, Cherry feels guilty about the way things have turned out for Pony and Johnny. Though she defends Bob and tells Pony about Bob's good-natured side, Cherry knows that it was Bob who savagely beat Johnny, and she must know that the greasers' side of the story about what happened in the park is true. She already knows that Bob was drunk that night and that he must have tried to hurt Johnny and Pony. She probably feels somewhat responsible for the boys having to hide out from the cops and for the injuries Johnny has received. With Bob dead, she has no true allegiances to the Socs, and she probably realizes that she might actually prevent more serious injuries by letting the greasers know that the Socs don't plan to use weapons. In the end, it is a way of Cherry to sever her ties with Bob's old friends; a way of showing her friendship for Pony; and a way to acknowledge that Pony would do the same thing for her.
"I wasn't trying to give you charity, Ponyboy. I only wanted to help. I liked you from the start... Do you realize how scarce nice kids are nowadays? Wouldn't you try to help me if you could?" (Chapter 8)
You answered your own question, but she also thinks the Socs are wrong.