What happens if I use an effect like Unlicensed Armory to prevent the death of the friendly unit I chose to kill with Baited Hook?

Baited Hook kills a unit, then uses information about “the killed unit.” If you prevent that unit’s death, there’s no killed unit, so Baited Hook can’t evaluate its Might. As a result, the check for the Might of the “killed unit” returns a null value, and you’re not able to play any unit. If the unit is killed before the ability of Baited Hook resolves, the outcome is the same.