At what point was Severus Snape’s greatest test of loyalty, when Lord Voldemort might have finally discovered the Potions Master was really working against the Dark Lord?
When Snape faced Voldemort in the Shrieking Shack at the end of …Deathly Hallows.
That is when Severus had to come to final terms with himself for all he had done, as he realized his life was about to end as the former Tom Riddle mistakenly thought he was trading Snape’s life for the allegiance of the Elder Wand.
“The Elder Wand cannot serve me properly, Severus, because… …The Elder Wand belongs to the wizard who… …killed Albus Dumbledore. While you live, Severus, the Elder Wand cannot truly be mine.” “My Lord!” Snape protested... “It cannot be any other way,” said Voldemort. “I must master the wand, Severus. Master the wand, and I master Potter at last.” Harry Potter and the Deathly Hallows – J.K. Rowling
There were several moments when Snape’s secret stood behind the paper-thin wall of his Occlumency; the first was when Severus agreed to spy for Professor Dumbledore, for neither Snape nor Dumbledore could be sure just how adept an Occlumens Snape was.
As Alex Forbes and Valeria Mesalina suggested, in the moments after the Dark Lord’s reincarnation in the cemetery in Little Hangleton, Snape was most certainly under heavy psychological pressure from Voldemort.
Severus would have played that scene over and again with Dumbledore, preparing for the possibility, but there were no guarantees; Voldemort might have killed Snape anyway, just to teach the other Death Eaters who again ruled the roost.
On the evening when Charity Burbage begged Snape for help at Malfoy Manor, the desire to rescue her would also have wrought havoc on Snape’s conscious; he could only watch one more whom he could not save, at least not at the risk of giving the secrets of Harry’s protections away.
But it is probably when he knew his life was actually coming to an end that Severus was most at risk of giving up his role as a turncoat to Voldemort; when his life was finally and clearly to be forfeited.
He might have given in to despair, could have felt that it was now all for naught, that Harry could not possibly survive now, without Snape’s aid.
Instead, Severus kept his psychological defenses up, gave himself up completely in the name of saving the Boy Who Lived.