While it might not be fun to find out that someone in your family or family history might be a criminal, if you need to check out information on someone, you might consider the US Criminal Databases.
JournalismNet provides a fairly comprehensive listing of most of the public US Criminal Databases with free access, or lets you know if there is a fee for access. It also includes information on finding property records, marriage and divorce records, death records, sex offenders, criminal records, and other public records, criminal and otherwise.
Searching criminal records is not easy, and to help you there is also an article on Navigating the Maze of Criminal Records Retrieval By Lynn Peterson to help you through the complicated process.
You never know where a good tip or information may come from when searching for someone or your family history and genealogy. ;-)