"My Lord, I have betrayed you." Snape knelt before Voldermort, who was seated on his throne (which was a transfigured Lucius Malfoy) in Malfoy Manor.
The Dark Lord's eyebrow rose. "Go on..."
"I am a spy for Dumbledore's Order of the Phoenix. I did it because you killed Lily Potter, whom I was in love with."
"Severus, I am not amused."
"I am sincere, My Lord. You may look into my mind to see for yourself."
"Severus, I am really not amused." The Dark Lord's tone had become dangerous. "You are not the type to play games."
"Look for yourself, My Lord." Annoyed, Voldemort did so, and found that Snape was indeed speaking the truth. Simple curiosity prevented him from immediately putting Snape under the Cruciatus Curse.
"Your death will be long and slow, Severus. I really must know why you told me this before I begin, though."
"The Marriage Law." was Snape's only answer, and all Voldemort needed to know.
"Ah, of course. Well, I suppose I can grant you this small mercy."
Snape's death was indeed long and painful, but it was better than bearing Potter's children.