What is the expiation for reporting someone who deflated another person's car tire, knowing that incitement to discord (fitna) is worse than killing, and is my action considered a mistake, and when is reporting someone considered incitement to discord (fitna)?

The fundamental principle is to inform the car owner about what happened without identifying the perpetrator, along with the obligation to repent, seek forgiveness, and never repeat such an act. It is permissible to identify the perpetrator if doing so becomes necessary for precautionary measures.