Severus Snape was a smart man.
In his life he had done many questionable things – but all his choices stemmed from the best intentions. He joined the death eaters before he was even out of Hogwarts, but his time at school wasn't as memorable as other students. He was bullied relentlessly, and unlike most of his Slytherin classmates, he did not come from money, nor was he a pureblood. He sought out powerful friends that needed him by his side. Lucius Malfoy was easy to manipulate, he already had an entourage- none of whom were very bright. Severus had a knack for potions, spells and course work came easily to him, and he was exceptional at occlumency.
Then Severus met Regulus Black- who unlike the rest of his friends was not proud and spiteful, and unlike his brother he wasn't a bully. Regulus was the friend that Severus always wanted- and one it turns out he needed. Regulus joined the death eaters as soon as Severus did, and was always quick to volunteer for things that Severus was too afraid to participate in. He was always there to step in, others saw him as power hungry, but Severus saw him as selfless and noble.
When Severus joined the death eaters it wasn't long before he realized the power and safety he had sought out in them wasn't what he was looking for. They inflicted pain, fear and suffering. They used and abused their powers against muggles, witches and wizards. Taking the mark was not something that was easily undone, there was only one way out- and Severus was not ready to die.
When Regulus died, Severus got scared, he along with a few others were taken under the Dark Lord's wing, and when Peter Pettigrew came to the Dark Lord with word of a prophecy Severus realized he had a chance to save the first friend he ever made. He found that the path he had chosen for himself didn't have to be all dark. He could save Lily Evans, he could try and stop the Dark Lord.
Of course, nothing ever happens the way you want it, Lily and James died, the Dark Lord went into hiding, and Severus positioned himself under Dumbledore's protection… waiting.
For years Severus thought he was waiting for the Dark Lord return, but instead in 1992 he came across Ginny Weasley – who was the spiting imagine of his friend Ginevra Dunham. He watched and waited, thinking this was cruel trick, Ginevra disappeared after Regulus's death and he had to mourn the loss of his two best friends, only to mourn the loss of Lily Evan's a few years later.
At first he had thought that Ginny Weasley was Ginevra Dunham's daughter. It had been a long time since the Weasley family added a daughter to their family tree, so it was possible- but there was no denying her resemblance to the Weasley's. The more Severus watched her, the more he realized that Ginny Weasley and Ginevra Dunham were one and the same.
Severus Snape liked to believe he was a smart man. Yet a smart man would have made the connection earlier, and would have confronted Dumbledore earlier. A smart man would have realized that Ginevra Dunham was not who she said she was. A smart man never would have joined the death eaters.
"Professor?"
Severus looked up from his desk, and was shocked to see Harry Potter standing in front of front of him.
"What is it Potter?" Severus asked.
"We got the basilisk fangs," Harry said, lifting his bag up.
Severus could hear a rustling sound, when Harry shook the bag for emphasis. Severus was suddenly annoyed. Harry was just like his father; he couldn't listen to direction.
"I asked for you to retrieve them when I had time to go into the chamber with you, Potter if you- "
"I know what you asked, but Ginny is Ron's sister, she was my friend as well as Hermione's," Harry countered, cutting him off.
"I'm well aware of what you three think of Ginevra, this is about your blatant disregard for my specific instructions."
"We want to help, and if this will help find her…"
"And you want me to keep a bag full of venomous fangs in my office Potter? So that an unsuspecting student can come across them."
"I err- didn't think of- "
"Exactly Potter, you didn't think," Severus stood up, reached out and grabbed the bag from Harry's hand. "Now get out of my office before I give you detention."
Harry turned on his heals, and walked out, Severus could make out some low murmurs when he stepped into the hall which he could only assume were from Miss Granger and Mr. Weasley before he heard their retreating footsteps.
He scribbled on a piece of parchment, and waved his wand over it, watching as the parchment disappeared.
Severus was a smart man; smart men learn from their mistakes. That's why Severus knew his best chance of finding Ginevra was working along side Sirius Black and Remus Lupin.