Yeah, the current mechanic of "you have to complete scanning them, before you attack them, even though they've just lased away all your shields and are wanted in 10 systems" is just terrible.
If they're wanted, you shouldn't have to scan them, period.
But you don't know they're wanted until you run their ship ID through the Galaxy database.
Unless you're psychic. Are you?
You know, the whole wanted system is pretty easy, but By God, I'd hate to take the whine and thunder out of you (and everyone else who complains about it) by proving it. Yeah, I'll do it anyway.
1) First/local scan, using the antennae affixed to the front of your ship (which is why you have to face them) is short-range and lets you know if they're wanted in your current system or not. If they're not wanted, then you will receive a wanted tag unless they fire first.
1a) You scan first and they're Wanted in the local system. Fire away!
1b) You scan first and find out they're not wanted in the local system. Run the Kill Warrant Scanner now, or if already being shot at, boost shields and fire back.
1c) They scan first and fire away at you, which definitely gives them a wanted tag. You still have to finish the first/local scan.
2) Dance the tango with them. Get some shots in, and when they pass you, or turn away, activate the KWS and run the Galactic scan. Sure, you could be blowing them away right now, but withou the KWS completed before the time you blow them up, you won't know if they have any additional bounties. So use this "down time" where you can practice staying on their 6 to KWS them.
3) KWS is done. Blow them away.
It really isn't hard. And it makes perfect sense. In fact, the mere idea of a person getting galactic data back after 10 seconds in deep space is fairly ridiculous. But for gameplay's sake, we, the players, accept it.
PS- If you submit to interdiction and they're behind you, put full pips in engines and boost twice, at least. Then turn around and target them. By this time they're ticked off at you and are shooting at you anyway. Put full pips in shields and run the KWS at the same time the local scanner is going. If you two pass, turn quickly and keep on him to finish the KWS.