In my opinion that would not be fair since there is nothing in the rules that says that last request prisoners are allowed there.
1. Freeday situations
Since alot of people accidently give out a freeday without noticing and thus killing everyone because they think their opinion is right and the Admin's fact is untrue (I've got these situtations so many times), it is really needed to be mentioned.
Whenever a new round begins and the warden opens the cell doors BEFORE giving the prisoners a command, the following round WILL end in a freeday. Also if the warden has no mic / does not give another command after 1 minute it's going to end in a Freeday.
2. Restricted Areas
Prisoners may not enter the guard spawn AND the armory UNLESS the warden says otherwise. Guards should never be accused of freekilling if the killed prisoner was in the guard's spawn or the amory.
If the warden gives the prisoners access to either of the 2 locations, any guard killing the prisoners in the allowed location can, should and will be punished by the admins.
As you see there is nothing in the rules about it. So in my opinion punishing a guard for killing a last request prisoner in the armory is not considered a freekill.
But as i said, if the main jailbreak admins agree that when it is freeday prisoner's are allowed to go to the restricted areas aswell it should be in the rules.