For clarity, here is how the Kill Warrant scanner *should* work (leaving aside bugs for a moment):
The KWS cross-references various authority records across multiple systems. The upshot is that when you complete a KWS scan, you will learn the combined value for all bounties issued against the ship.
You can check this value in the contacts tab of the target panel (the panel you have to UI focus on, not the basic target panel): just select the ship and it will display the combined value.
When you destroy the ship, this combined value will be split up into the individual bounty claims in your contract tab; for example, a 5000k combined value might turn into a Federal claim for 2000k and an independent system claim for 3000k.
Importantly, the KWS *does not* give you authorisation to attack a ship. It merely reveals how much the ship is worthy in bounty claims, in total.
It is perfectly possible to have a ship that is clean in the current location, but wanted elsewhere. Attacking a ship that is clean will result in a crime being committed, unless you are attacking in an anarchy (where no crimes are logged).
On the other hand, just because you get a bounty for attacking a ship that is not wanted in the current location, you are not prevented from cashing in any bounty claim you get for destroying it. Various factions that issue bounties don't really care what laws get broken in order for the perpetrator to be brought to justice as long as their own laws are respected.
The bottom line is: unless you are in an anarchy, you will be committing a crime if you attack a ship that isn't wanted, or that you have not completed a basic scan on. A KWS will allow you to potentially earn lots more money by allowing you to cash in bounty claims from systems other than the one you are currently in.
Of course, the fly in the ointment here is that there are a couple of bugs.
Bug number one is that when a ship attacks you, your interface will tell you that it is wanted, but it is actually fibbing – you still need to complete a basic scan to avoid getting a bounty. This has been fixed, hopefully to deploy in a build soon (I have a particular hatred of this bug).
Bug number two is that a KWS result in super cruise is lot if you interdict the ship – you have to complete a KWS in normal space to get its benefit. This is a lower priority issue for us at the moment, so the fix will likely be post release. However, as has been stated many times, development will continue after release, and not just for expansions but for a whole bunch of stuff we want to add and finesse.
If you perform a basic scan on a ship (target the ship and face it until its criminal status displays (which will be "clean" or "wanted")) and it shows as wanted, that means you can legally attack the vessel without penalty. If it comes back as clean, then you will need to wait until the ship successfully attacks you (at which point its criminal status should change to "wanted").
If you use a K-warrant scanner and see that a ship has a bounty (by looking at the target side panel, tabbing to "contacts" and highlighting the ship) you still need to make sure that the vessel is displaying "wanted" on the target information pane – the K-warrant scanner checks for crimes in all systems, so the ship could be wanted elsewhere but clean in the current location.
If this is the case, you would get a bounty for attacking it, so you'd need to either A) take the bounty hit ('cos you'll still be able to claim the ship's bounty in other systems), B) let the ship attack you first (at which point it will get a bounty for the current system... as long as YOU don't have a bounty in the current system!) or C) follow the ship to a system where it is wanted, or an anarchy (where no crimes are logged either way).