In Elder Sign, If I free a locked die by defeating a monster, can I use it in the same turn?
Not normally - monsters defeated within adventures are not destroyed and removed until the end of the adventure.  Theoretically, this would work if you could destroy a monster instantly with some sort of monster-destroying special effect in the middle of the adventure; however, I believe that all such effects have timing windows that intentionally exclude the ability to do it at that time.