"Has compunctions against" is not the same thing as "will therefore disobey a legal order, or the law." A good person would work for the good of other people and break the law if necessary. An evil person would be more than willing to harm the innocent using the law as a weapon, as Tarquin does. But to an LN person, neither of those things will happen. An LN person would not use the law as a weapon, but neither would they defend an innocent person being targeted by such means. To them it is not about good or evil but about Law."Has compunctions against harming the innocent" is a general trait of non-evil alignment- not just Good. A Neutral person given an order that qualifies as "harming the innocent" - even a legal order- will at least have compunctions about obeying it.
And thus an LN Malack would be the perfect henchman and counterpart to the LE Tarquin. Someone dedicated to law above all is the perfect willing executioner for the one who makes the laws for his own ends.
ETA: This may be where D&D alignment breaks down. We call Malack evil because in OUR world's view he WOULD be evil; real-world ethics don't recognize a neutral alignment. But D&D does. And in D&D a neutral character can definitely be used as a tool for evil by someone who is a great deal more intelligent than they are. And Tarquin certainly seems to be the Alpha as far as brains go in their friendship.
Respectfully,
Brian P.