*FOUND!* Stiles is with the Argents - runs away to warn the Hales
Stiles is living with and possibly raised by the Argents and trained as a hunter. He knows that what the Argents are doing (murdering supernaturals for sport) is wrong. Stiles learns that the Argents plan on attacking the Hales and runs away (with Allison?). I think he has a car hidden that the Argents are unaware of and he races ahead of the Argent hunters to warn the Hales.
I might be mixing two fics but I believe the fic starts when Gerard kidnapped Stiles when he was young and maybe killed his parents. Stiles remembers his family fondly so every time Gerard tells him something awful Stiles reminds himself that the man is a bad person.
That's all I remember. I know I read it on AO3. Unsure of any pairings.
TIA.
Found after hours of searching and thinking I'd lost my mind:
Silver and Fire by Triangulum
https://archiveofourown.org/works/11872164