Severus Snape was killed on May 2nd, 1998, by Voldemort. Voldemort killed him because he thought that Snape was the master of the Elder Wand, and if he killed him, then he would be the master.
Snape is a half-blood. He did not have a good childhood. It is possible that he was neglected at home. Also, he was bullied by James Potter and Sirius Black from the moment he started attending the school. Taking the bullying to far once, Sirius almost led Snape to a certain death. Sirius told Snape about the hidden passageway under the Whomping Willow, which led to the Shrieking Shack. There Remus Lupin would be, in his werewolf form. Either Snape would be killed, or he would become a werewolf himself. Luckily, neither happened because James stopped Snape from going down the passageway.
Throughout his time at Hogwarts, Severus didn't have many friends. His one true friend was Lily Evans. He has known her since he was nine years old, since she lived nearby. He told her all about magic, because he saw her using her magic before she even knew what it was. They were the best of friends. At some point in time, Snape fell in love with Lily. However, she never returned the feelings. Worse than that even, he lost her as a friend completely. In their fifth year, James Potter had been bullying him as usual. James had used the spell "Levicorpus" on him, which ironically, Snape invented. While Snape was suspended in the air, Lily came to the rescue, yelling at James to stop it and to put him down. However, Snape did not want her help, and he called her a mudblood. Since then, Lily would no longer be Snapes friend, and it would be his biggest regret.
At some point in time, Snape became a Death Eater. However, some time before the Potters were killed he turned spy and was also a member of the Order of the Phoenix. When he found out that Voldemort was to kill the Potters, he begged him not to kill Lily. His begging was in vain, because Voldemort killed Lily when she refused to step out of the way of Harry.
Severus became the potions master at Hogwarts. During Harrys' time there, it was his job to protect Harry. Although Harry actually believed that Snape was working against him, he was not. One example is in Harrys' first year. Quirrell tried to curse Harry when he was participating in a Quidditch game, but Snape was doing the counter-curse. The trio believed that it was Snape that had done the curse, and there had always been a mutual hatred.
Through everything, Snape still protected Harry and he still loved Lily, even though she had been dead for years. Severus is the one that helped Harry find Gryffindors Sword, he is the one that sent the silver doe patronus. Lily's patronus had been a silver doe, and Snapes patronus being the same showed how much he truly did love her.
Voldemort ordered Nagini to kill Snape. However, he had never realized that Snape was really working for the Order. He hadn't even realized there was a spy. Pulling something like that over Voldemort is impressive. Snape was incredibly intelligent. And he was also brave, because if Voldemort ever found out he knew that he would be after him. Also, even though no one had ever loved him, he knew how to love. His love was so deep for Lily, it was so strong that years after she was gone he still loved her all the same. It's unfortunate that Harry did not know of any of this until Snape was gone. But he did honor him, in naming one of his sons "Albus Severus". He also made sure that Snape was remembered by getting a portrait of Snape in the Head Masters office at Hogwarts. As Harry remembered Severus Snape, we should remember him, for his bravery and everlasting love.