Formal:
* Perpetrator: This emphasizes the act of committing the crime.
* Offender: This is a general term for someone who has broken a law.
* Transgressor: This implies a violation of a rule or moral code.
* Delinquent: This suggests a minor crime or wrongdoing.
Informal:
* Guilty party: This is a direct equivalent to "culprit."
* Miscreant: This implies someone who is mischievous or wicked.
* Wrongdoer: This is a general term for someone who has done something wrong.
* Sinner: This implies a moral transgression.
* Troublemaker: This emphasizes the act of causing problems.
* Scoundrel: This implies someone who is dishonest or immoral.
The best synonym for "culprit" will depend on the context and the specific meaning you want to convey.