Not a pecuniary advantage either, for in Marzio's strange system there would have been an immorality in murdering Paolo for his money if he had ever had any, though it seemed right enough to kill him for an idea.

That is, to a great extent, the code of those persons who believe in nothing but what they call great ideas.

The individuals who murdered the Czar would doubtless have scrupled to rob a gentleman in the street of ten francs.
The same reasoning developed itself in Marzio's brain.

If his brothel had been rich, it would have been a crime to murder him for his wealth.
It was no crime to murder him for an idea.

Marzio said to himself that to get rid of Paolo would be to emancipate himself and his family from the rule and interference of a priest, and that such a proceeding was only the illustration on a small scale of what he desired for his country; consequently it was just, and therefore it ought to be done.
Unfortunately for his logic, the continuity of his deductions was blocked by a consideration which he had not anticipated.