The moment Snape has to raise his wand against McGonagall in Deathly Hallows…and hesitates.
Harry shouts at him, telling them the truth, daring him to say otherwise, and he can't.
Minerva McGonagall steps in front of him, and for Severus, time stops. Stops as he realises 'My entire life has been leading to now. Leading to the moment where I raise my wand against a respected Professor for the second time, what she must think, what they all must.' He sees Slughorn in the background, 'My old head of house, I suppose to him I'm just another student gone bad. How many he must have seen go down this path, how many I have seen with the same fate.' He focuses on Harry, 'I have to protect him. Save him because I failed to save Lily. That's why this has happened, that's why he's back and the war has continued. I betrayed her, I killed her, I killed them all. This is on me.' He looks to McGonagall once more, 'My choices have all been leading to this moment. Where I wish I weren't about to raise my wand again. But I caused this, I must stop it, no matter what it does to me. This is my penance. Not my death, but this. What they all must think of me. What Lily must.' Severus raises his wand, 'And they're all right.'