Applying this to Snape, we know from previous experience that he has his emotional weak spots. His reaction to Harry's intrusion into his memories of humiliation at the hands of James Potter and ...

The final Harry Potter films reveal Snape's true nature in the form of memories the teacher gives to the Chosen One. There, he relinquishes his secrets, showing the deep and devoted love he had ...

As Lucius's old friend then, or for some motive only known to himself, Snape agrees to make the Unbreakable Vow to Narcissa, using his own life as collateral. Just to refresh our memories ...