A gun’s purpose is to kill people, a car’s is not. The analogy is flawed.
Still, assuming you have mandatory regular inspections of cars in the US, imagine you are an experienced mechanic by profession. Someone lends you a car and says it’s safe but you know immediately this rustbucket hasn’t been to an inspection in decades. By experience and papers. But you drive in a public space anyways and kill someone due to a fault that would have been found during an inspection. It is 100% your fault.
As I understand it, following safety procedures would have prevented this death, in the same way nonfunctional brakes would certainly be found during service.
On a side note, as an electrician who has to sign documents that electrical devices are safe to use, if one of those devices kills someone and I can prove that I followed protocol during testing, I am in the clear. Following rules makes the difference between a tragic accident and negligence.