If they aren't Wanted in the current system (so a Wanted label from standard sensor scan), and you aren't in an Anarchy system, then opening fire on them will earn you a bounty. The KWS is not a license to kill as far as the local authorities are concerned.
Think of it this way: you are in Federation space, scanning a Federation aligned ship. The KWS lets you know that the glorious Empire has opened a bounty on the ship. Obviously you should kill him, but the Federation will take a dim view of this act and will place a bounty on your head.