Keep or Slaying Edge+ fodder?

Palla is really bad. Use her for fodder and maybe give it to a unit like draug and give him bonfire. He will make a great knight unit until you get stronger reds. You could even give the killing edge to Lon'qu. He has fantastic speed and will double many green units. Give him a decent special and you might end up with a unit almost as string as marth, lucina or even lyn

I would not sacrifice a 5* for a 3 atk increase on an otherwise already complete hero. I would rather give it to a 4* merged unit like olivia that does not have access to a 5* weapon. The other swordsmen you named would not benefit greatly from the slaying edge+ either. I would probably just keep Saber, hes pretty useful.

Do you have Wo Dao+ fodder available? I think that Moonbow +10 DMG would take you further than two charge Luna aganist anything but armor.

If not, then Slayer's Edge is a good option. Personally I wouldn't run Saber, if Palla doesn't get it then someone else would make better use of it (like Gray - your other sword units have better legendaries).

I'd Keep the Saberfor potential new units who can make better use of slaying edge. (not that palla wouldnt)
I dont like wasting multiple 5* weapons especially if the one she has is pretty much identical except for a few extra MT.

Also i feel that with buffs (assuming u have hinoka or hone fliers) it kind of makes up for the difference in MT from killing edge and slaying edge.

Palla's best weapon IMO is Wo Dao+, id hold out for that weapon and hold onto saber and see what new units u get in the future.

Hope this helps