From this article:

Separately, the Federal Aviation Administration acknowledged violating its own policies when it assigned only one controller to the Lexington tower.

The reason the FAA established a policy requiring two controllers is that the FAA itself foresaw that having only one controller on duty is not enough to ensure the safety of travellers. Whoever approved operating the Lexington tower with only one controller ought to be fired or sued since that decision resulted in the death of 49 people. The way I see it that person either:

a. did not know the FAA rule regarding staffing; in which case, that person is incompetent.

b. knew the rule but disregarded it; in which case, that person was criminally negligent by knowingly putting the lives of travellers at risk.