An employee can be justly fired for violating safety policies.

In this context, it can be argued that the harm of being injured/killed outweighs the harm of being unemployed.

Yes, it is a valid argument in this case. However, the court has to consider the strength of that argument in relation to other arguments.

There are numerous aspects of American employment law wherein the harm of unemployment may be considered. For instance, it is relevant in cases of employment discrimination, and many states also have laws surrounding acceptable reasons for termination of employment. It is clearly not a priority of the legal system, but it is sometimes a relevant consideration.