Severus Snape is an observant man. He has had to be.
When he was young he learnt to observe his father's mood, to know when it was best to get out of the way. At school he was always watchful or Potter and Black would be ready with some prank.
He watched for his Lord's displeasure, for the subtle clue hidden in the sibilance. He watched political tides change and, later, for signs that the same Dark Lord suspected his treachery.
Observance was a great asset in Slytherin House, where lies and truth mixed equally. A fine skill he had honed even sharper as a teacher, aware of the misbehaving children and the constant mischief of Harry Bloody Potter.
Still, he was surprised to see that, while those who fought regrouped at the school after the battle at the ministry and the death of Sirius Black, no one else noted the absence of Remus Lupin.
Albus Dumbledore was also an observant man. He watched Snape leave.
Snape heard the screaming only after he had opened the door. As the flames attacked the picture, the paint, Black's mother's very skin, was blistering and peeling as the woman screamed.
He took a moment to reflect on the physical destruction, which had prevailed where charms and magic had failed. The canvas, the woman's flesh and bones, her captured soul, was exposed and consumed. He charmed the walls around the picture and let her burn.
As her screams died he heard another living scream as living flesh was destroyed. Snape followed the sound up the stairs and into the master bedroom.
The small body of Black's house-elf was being flung, torn and broken by a madman. The resilient creature was still alive, continuing to screech after hitting the wall at and angle, Snape knew, that would have broken a human's neck.
Lupin dug his fingers into a tear he had made in the skin around the elf's neck. Soon the creature 's screaming was muted, distorted, burbled as it choked on it's own blood. He was not entirely surprised when the head finally came off.
He saw Lupin's face. The man was never more the wolf than at this moment.
Severus Snape is an observant man. He looked at his enemy, his brother in arms, and wondered why he hadn't noticed before.
He raised and eyebrow, sneered, and left.