It is 100% fair to expect people working on life-critical systems to either a) know that they are competent to work on life-critical code, or b) to not work on life-critical code.

The guy wasn't ordered to do this at the point of a gun. His bosses asked him to do something, he said yes because he liked the money, and people died. The bosses are also responsible, but that doesn't mean that his negligence didn't kill people.