SAN DIEGO — Guns were the most-used weapon in homicides in San Diego County last year, the assailant usually knew the victim, and arguments were the most common motive in the killings, newly released research found. Released Tuesday, the data review by the Criminal Justice Clearinghouse looked at victims and suspects in violent and property crimes in the region in 2021. Clearinghouse is part of the San Diego Association of Governments. The Clearinghouse report found racial disparities among both suspects and victims of violent crimes. Black people make up 5% of the county population, but accou…