Not quite, you’re ignoring the role of the armorer on set in your metaphor.

If you just picked up your car from the mechanic after they were expected to check everything, including the brakes, and the brakes then fail causing you to crash and kill someone… Is it manslaughter? And if so, who is at fault?

You were driving the vehicle, but you would obviously expect the brakes to be in working order since they were supposedly checked immediately before you started driving. The driver would almost certainly not be charged in that case, but the mechanic on the other hand would clearly be negligent, directly leading to the death.