Severus Snape being bestowed a Chocolate Frog trading card due to his "heroic" double-agent spying struck Lucius Malfoy as nearly inconceivable. The Dark Lord chose Lucius as Severus' mentor within the Death Eater ranks while the younger wizard still attended Hogwarts. During that time, a bond formed between the two men, and when Draco came into the world, Lucius could only imagine one person worthy of the title of godfather to his heir.
Time and again, Lucius used the Potion Master's teaching position at Hogwarts to his advantage while serving on the Board of Governors for the school. Severus passed him insider knowledge, making sure Lucius rarely found himself surprised. Severus expected, and received, quid pro quo. When unforeseen events did arise, Severus swore to him they were due to Severus' own lack of knowledge. Lucius now doubted the veracity of that.
During his time at Hogwarts, Draco had been quick to assure his father that Severus treated Potter with the utmost disdain in every class. His son gleefully reported that although the Mudblood best friend of Potter attained top marks in their classes, Severus found ways to belittle her constantly. Draco also spoke of the favors and concessions his godfather granted to him.
Lucius' trust in Severus led to Narcissa's own confidence in the wizard, culminating with their unbreakable vow while Lucius remained locked away in Azkaban. Lucius idly wondered how quickly Severus ran, black robes snapping behind him, to report that particular event to Dumbledore?
The Dark Lord looked into Severus' mind and only saw loyalty. How did the man fool their master? How was Severus able to hide his continued love of Lily Potter and the desire for revenge for nearly 20 years?
Even on that fateful day, The Dark Lord hadn't killed Severus for his hitherto unknown disloyalty; his murder was seen merely as a ruthless means to an end.
A rumor reached Lucius a couple of years ago that none other than Potter had been in possession of Severus' old potions textbook for a time and that it contained a plethora of revolutionary tweaks to current potions, original potions created by Severus, and spells invented by the wizard. Lucius judged the contents of those pages would have been worthy of recognition. But, The Idiotic Man Who Lived allowed the book to burn in the Room of Requirement along with that truly idiotic boy, Vincent Crabbe.
Lucius took one last look at the trading card of the blood-traitor he once believed to be his closest associate, pointed his wand, and declared, "Incendio!"