More of the same. Feel free to do what you like in your universe. I interpret the Imperium a bit more toward the Orwellian side of the equation. The Imperium has been around quite a long time, and its navigational laws are well-established. All sorts of Rogue Traders have done exactly what you describe in the past, causing all sorts of anarchy, and so the laws have become more and more prohibitive (closing the loopholes, so to speak). And no, those laws aren't enforced outside the Imperium, but at some point your boy comes home and his actions follow him.
Javcs, my example of the battlebarge was a facetious attempt at pointing out the absurd, not a real example. It was the extreme limit of where this reasoning eventually leads.
And Blade, your side of the argument is the one that has argued itself into a corner. Your interpretation rests on "what happens outside the Imperium stays outside the Imperium." No it doesn't. Our world isn't that simple and the galaxy of 40k is scales of magnitude more complicated.
About 5000 years ago, the first laws were invented for just this reason. People, perceiving injustice done to them, would take the matter of justice into hand. Then another group, seeing themselves wronged in the matter, would also join in. Before long anarchy reigned and the populous was unhappy. Strong men took control of the chaotic populations, backed by other strong men, specialized artisans, and the educated. States were born. Civilization was thus created (advanced social institutions, usually referring to law and religion, being a prerequisite of civilization). Your theory leads backwards...to an absence of civilization. Your theory backs the argument that the Imperium of Man cannot exist. That's a corner and you're standing in it.
Baron, I haven't overlooked a thing. You do me a disservice to think I haven't considered all facets of a Warrant of Trade. Yes, the Warrant grants astounding powers and privileges, even the power to declare and wage war, and even drag the Imperium of Man into that war. It does not, however, grant the Rogue Trader the power to war on the Imperium, and other Rogue Traders are representatives of the Imperium quite the same as your Rogue Trader is. When you attack another Rogue Trader you will eventually be called to task, just as an Inquisitor gets called to task when getting into a planet-devastating squabble with another Inquisitor. Then it comes down to a matter of legalities and the details surrounding the incident in question. The OP quite explicitly said they attacked the rival RT "just to f*** with them and for the fun of it all (paraphrased)." I have a hard time believing that will hold up under the scrutiny of investigation. The detectives of your universe might be incompetent but not in my universe. Your example of an RT engaging another to stop the Cold Trade WAS a good example of one of the incidents one might use to cover what was essentially an attack of opportunity. Once again, the OP did not even give a hint that this was the case. In fact, a cover-up was never suggested and the entire point of the post was to determine the amount of "loot we get from this dungeon."
And lastly, javcs, exactly what kind of dirt is that rival RT going to dig up on the original? Piracy isn't against the laws of the Imperium in your universe. In our own little world murder, ****, and arson are usually considered the "Big Three." They aren't even illegal in your universe. I'm guessing heresy, blasphemy, and xenos-dealing are your "Big Three." And that's fine. I'm just not going to buy into the notion that the others have disappeared as capital crimes.