Just before Lily Evans was murdered by Lord Voldemort, Snape changed sides and became a member of the Phoenix Order and double agent during the Second Witch War. With enormous difficulties, Snape prevented Lord Voldemort from knowing the truth about his allegiances. Despite the opinions of most others, including Harry early in his life, Albus trusted Dumbledore Snape for reasons that were kept between them until their death. Although Snape killed Dumbledore, they learned they had a special agreement to do so. When he died, we learn that his deep love for Lily Evans led him to redeem himself by associating himself with Dumbledore`s cause for his protection (and after his death, that of his son) of Lord Voldemort.  From 1971 to 1978, Severus attended the Hogwarts School of Witchcraft and Wizardry and was incorporated into Slytherin House under the direction of master gossip Horace Slughorn. On his way to Hogwarts for her first year, Severus was on the train with Lily Evans. On the train, they met James Potter and Sirius Black. This meeting between the three revolved around a disagreement over what was Hogwarts` best home. This first hostile encounter would set the tone for the three for the rest of his life.
 When the forgery was returned to Dumbledore`s office, Luna Lovegood, Neville Longbottom and Ginny Weasley made another attempt to seize the sword. After Snape claimed to punish her by sending her into the Forbidden Forest with Rubeus Hagrid, Snape handed the fake sword to Bellatrix Lestrange, who kept it in his vault near Gringotts, thinking it was real. It seems unfair that Dumbledore did not have to keep Snape confident about Harry`s likely fate. For someone who had put his life in danger to keep Harry safe, to discover that the boy had never thought of surviving, and that Dumbledore may have known him all the time, or at least much longer than him, would of course have felt like a betrayal. Severus became a member of Hogwarts of the Death Eaters. During his schooling he was a member of a “band of Slytherins, who later became stenatis.”