Even if he had been convicted of a misdemeanor it wouldn’t have been enough to disqualify from buying the gun. If the family he was living with had prosecuted for domestic violence, on the other hand, that would have been. So not only the fault of law enforcement by a long shot.
Some of Nikolai’s offenses went beyond misdemeanors:
Assault his brother.
Assault his mother.
Assault neighbors.
Stealing US mail.
Trespassing
Vandalism
Voyeurism
Cruelty to animals
Destruction of private property
Throwing rocks at moving cars
Threatening to kill other people