“While I will admit I have questioned many of your decisions,” Snape spoke at last, “You do generally employ sound judgment… You do not make the decisions I would make, but your choices are often not without merit.”
“I went to rescue Sirius,” Harry said darkly. “I was stupid. You wouldn’t have done that.”
“No,” Snape said flatly. “Nor would I have spared Pettigrew. I would have left Draco Malfoy to die at the hands of a Dementor. And if I’d encountered my most hated professor, suffering in the aftermath of torture, I would have ‘left him to rot’. My life would have been considerably easier in many respects… but I am not you.”
He leaned back and watched Harry speculatively.
“As you can see,” Snape concluded, “we are very different people, you and I. You dare to gratify your nobler impulses, and I myself have never had that fortitude. You rescued Miss Granger from a troll, and to this day she is your friend. Black,”– Harry noticed he did not quite spit the name this time,– “owed his life to you, and was devoted to you in return. We approach matters very differently, which is one of the reasons you’ll very likely live out your life surrounded by loyal friends, and I’ll very likely die alone.”
(IBO, Chapter 41)