You need to get 3 NPCs over the river, but the boat is only big enough for you and an NPC.
NPC1 wants to kill NPC2
NPC2 wants to kill NPC3
NPC3 is a pacifist
They will only kill the other if there are going to be no witnesses. However, you need them all to get out of the cave!
Get them all across the river, and there's your puzzle! (really lame puzzle, right?)