This is wrong then, the HUD should be only showing active threats ... and active threats are shoot-able it's self defense.
The purpose of a HUD is to show active threats, not make lawful judicial judgments.
The purpose of a HUD is to give you information. In ED, it gives you both legal information and information of the attitude other pilots have toward you. All ships are threats, and no ship that isn't currently shooting at you is really an "active" threat.
The scanner shows the attitude of ships to you. It does not show legal standing. If the ship is flashing then it's an active threat.
The target panel shows whether or not a ship has a bounty. It does show legal standing.
Attitude is entirely separate from legal standing. A green (friendly) ship can carry a bounty and therefore be a legal target. A red (hostile) ship can be clean and therefore illegal to shoot. If you want to avoid legal problems you must check your target's legal standing.
As for whether shooting hostile ships in anticipation is self-defense, that's highly debatable. Just because they are hostile does not mean they will shoot you, in fact if they are clean they usually won't. If you shoot first and they are clean you are the aggressor. It is right and just that you should be charged with a crime in that case. Shooting someone simply because they dislike you is not justifiable.
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Ethics is great and everything, but this is a really big point. ED has a lot of trouble being a game and this is one of the reasons why. Miscommunication tends to leave to easy to make are often hard to correct.
A player's failure to understand what the interface is showing isn't misinformation. And really the ED HUD is not complicated.