This is wrong. Privateers, which is the more general class corsairs belonged to, were akin to modern PMCs and operated under similar constraints. They mostly obeyed the rules of war, were punished when they did not, and their conduct was similar to national ships in nearly every regard. They took prisoners of war and were taken prisoner in turn.

When one group is taking ships and killing every living thing on it then illegally selling the cargo and personal effects of the occupants, and another group is taking ships and dropping the occupants off at a POW camp then sending it to the admiralty to be legally sold, the latter is not doing the same kind of robbery. Arguably they aren't doing any kind of robbery at all.

It was the colonial era, right? That a gang of thieves and thugs stole enough to afford fancy wigs and crowns doesn’t make them legitimate.

The legitimacy of a government, or lack thereof, does not make executing the entire crew of a ship morally equivalent to sending them to a POW camp.

I don't think pirates regularly executed the entire crew of a ship either. There's no monetary benefit in doing so, it will encourage reprisals, and future ships will refuse to surrender.

Once we drop the legality fig leaf, because that's a matter of who writes the law, we'll see an organized gang of pirates that follow some rules, wear an emblem, and have bosses on the land who sell things captured in the sea.