Most people are shot by someone they know.  Many of these shootings, and murders, occur within the family.  

A gun in the home is 43 times more likely to be used against the gun owner than it is likely to be used against someone from the outside coming into the home.  

Nancy Lanza, the mother of mass-murderer Adam Lanza, had her gun used against her, by someone she knew.  This possibility is described well in reality; it's more common than not.  What happened next is the statistical anomaly.  Mass murders are few, thankfully.  But mass murders need weapons of mass murder to happen.  Mrs. Lanza bought and kept such weapons in her house.  

Guns do not make the gun owner safe, then.  And weapons designed for mass murder will be used when mass murder occurs.