Presumably the developer in this scenario is not responsible for guaranteeing to and/or misleading a customer that his code adequately renders a deadly weapon safe, more likely that responsibility falls on the managers whose project it was to implement such a digital "safety" and instructed the developer to write the code in the first place.
Assuming the developer did not write the code with the intent of bringing about a person's death, which might involve fooling his superiors as to the efficacy of his work, I'd wager he can't be guilty of a crime. Outside of that, I believe vicarious liability applies.
Perhaps he was a one-man department of this hypothetical gun manufacturer, implementing, deploying, and marketing his product himself and of his own initiative; I don't think there's any doubt he'd be liable in this case, the extent of which is dependent on his intention.
Perhaps our developer was self-employed as a contractor, and warranted to his client (the hypothetical gun manufacturer) that the code was safe; assuming the manufacturer used it as warranted, and it failed, I'd assume the developer would be mostly liable for the result.