No. You're not. By that logic it's also your fault if someone steals your car and uses it in a robbery. Steals your bike and kills someone during the getaway. Locked is secured. You locked your front door, took keys from ignition etc. Gun shops don't even put all their guns in the safe at night. Most major chain retail gun shops don't even have a safe. No gun distribution warehouses have safes. Gun manufacturers don't use safes. That semi truck that's parked at the truck stop for the weekend loaded with guns for Monday morning delivery is not a safe. No, you're not negligent or liable