I believe that the theme behind Harry Potter and the Half-Blood Prince is “love”. On the periphery, we see Fleur planning to marry Bill despite his gross disfiguration. We see Tonks convincing Remus to love her and be with her, despite all of his self-perceived flaws. We see Harry fall in love with Ginny and see Ron fool around with Lavender, but eventually coming to realize that Hermione has always been the girl for him.
But ultimately we see a deeper love. Dumbledore tells Harry that he has what Voldemort does not – love. Harry had his parents and in fact lives because of his mother’s love. Indeed, when Harry looked in the Mirror of Erised, he saw his deepest desire – the love of his parents. That love, Dumbledore says, is Harry’s secret weapon. Voldemort had no one – his mother died shortly after childbirth and his Muggle father rejected him by abandoning his mother and never claiming him as his son. Dumbledore says that Voldemort liked Hogwarts because it was the only place he could claim as his – a place where the nearest thing to love existed.
Snape inadvertently killed Harry’s parents by telling Voldemort about the Prophecy. I believe that this incident changed Snape forever. He left Voldemort and became Dumbledore’s man. I believe he did so because he was repulsed by Voldemort’s lack of humanity – even miserable Death Eater Snape could feel love. I believe that Albus made a pact with Snape that Snape would become Harry’s guardian. That Snape would keep an eye on Harry and, if need be, do what is needed to protect him.
When Harry told Dumbledore about Snape’s pact with Draco’s mother, Albus continued to claim that Snape was on his side. After Dumbledore drank the poison in the cave, he knew that he was dying. He was already badly hurt from the curse he received from destroying Riddle’s Ring, but the poison in the cave obviously exacerbated his deterioration. When Dumbledore and Harry returned to Hogsmeade from the cave, Harry said that he would call Madame Pomfrey, and Albus said, “No, it is Professor Snape whom I need.” Dumbledore knew that he was dying of the poison and he wanted to remind Snape of his duty to protect Harry at all costs.
Remember when Harry had the Half-Blood Prince’s Potions’ Book and Snape knew it had been his old textbook. Snape could had called it from Harry’s pack with an “Accio” command or simply force Harry to give it back to him – but he did not. He told Harry to get the book, thus giving Harry ample opportunity to hide the book and substitute another. I believe that the potions’ book will help Harry out in his quest to kill Voldemort, and Snape knew that.
When Dumbledore is alone on top of the astronomy tower with Draco, he tries to convince Draco to come to his side – not to become a murderer. Draco says “I've got to do it! He'll kill me! He'll kill MY WHOLE FAMILY!” Draco said the same thing to Myrtle when he was crying in the girl's toilets (which Harry overheard). For the first time, Draco shows love. Draco is not a true Death Eater. Dumbledore wanted to save him at all costs – to put him in a “witness protection program,” as it were. Draco is terrified that the Dark Lord will murder his family and he hesitates. Enter all the Death Eaters who order Draco to kill Dumbledore. Draco can’t. Enter Snape. Snape had made a pact with Draco’s mother that if Draco cannot accomplish his mission, Snape would do it on Draco’s behalf. Dumbledore is dying. As he keeps sinking lower and lower, he looks at Severus and keeps saying “Severus… Please...”
One might think that Dumbledore is begging for his life. Not so. Out of love for saving Draco and keeping him from becoming a true Death Eater, out of love for protecting Harry who is hidden immobilized under the invisibility cloak, and out of love for Snape who will die if he does not fulfill the Unbreakable Bond that he was forced to enter into so as not to blow his cover, Dumbledore is begging Snape to kill him. Snape keeps hesitating, just as he hesitated to fulfill Albus' wishes when Hagrid overheard them fight. But in the end, Snape understands that out of love for Dumbledore he must kill him, so as to fulfill Dumbledore’s desire to save Draco, Harry and himself. And so Snape murders Albus and has to live with that.
I believe that in the end, Snape will try to save Harry, probably dying in his place to give Harry time to kill Voldemort. Remember – Dumbledore reminds Harry that the weapon that he has that Voldemort does not is love. That is the theme of the entire Potter series and is played out on many levels in this book – physical and philosophical.
There are other themes of love. Hagrid’s love for the spider, Aragog. He raised Aragog. When the spider was sick, he cultivated special grub worms to feed to him. Hagrid read Aragog stories at night to make him feel better. When Aragog died, Hagrid buried him with great love. Professor Slughorn’s milking the spider for his venom is a foil to show the difference between Hagrid’s deep love for Aragog and Slughorn’s use of the spider to make money.
Death and love are intertwined in this book. The death of the spider and Hagrid’s love for him. Dumbledore’s dying and all the wizards and witches that came to honor him at his burial. Contrast their love with the purely utilitarian attempt of Scrimgeour, at the funeral, to convince Harry to become the Ministry’s poster boy. Scrimgeour and Slughorn vs. the wizards/witches/Hagrid. And, as I noted above, Dubledore’s death was a death of love. He drank the poison, knowing full well that he might die, but drank it anyway so that he could save Harry as well as the Wizarding and Muggle worlds from Voldemort’s evil. Snape killed Dumbledore against his own wishes but out of love for Dumbledore – he fulfilled Dumbledore’s request even though he obviously did not want to. And even Slughorn came around at the end, risking his life by giving Harry the Horcrux memory out of love for Harry's mother and out of a sense of duty to finally do the right thing.
At the very end of the novel, Harry says that he is not coming back to Hogwarts. He says that he is going to his aunt and uncle’s house because Dumbledore asked him to. Then he is going to visit his parent’s grave and then commence a mission to find the four remaining Horcruces.
Out of love for Ginny, Harry leaves her to protect her. Out of love for Harry, Ron and Hermione say that they will also not return to Hogwarts in the fall and instead join Harry on his quest, even though they may both die in the process. Finally, Ron says that there is one more place they have to go before they visit Harry’s parents’ grave. They have to go back to the Burrow for Bill and Fleur’s wedding. And so the book repeats the cycle of love as it prepares itself for the final installment.
I believe that Harry will find out that he is the final Horcrux (the "Gryffindor" Horcrux) and realize that he ultimately must die in order to kill Voldemort. And it is out of the same love that Dumbledore held for Harry, Snape and Draco when he pleaded for death on the Astronomy Tower that Harry must ask his two best friends to kill him or let him die, for only in Harry's death will peace return to the Wizarding World, the only place that Harry has known love in his short, tortured life.
One last word. If Harry were the final Horcrux, and if he realized that he must die in order to destroy Voldemort, then J.K. Rowling would have turned Harry into a Christ figure. The Chosen One – he who must choose to make the ultimate sacrifice out of his love for mankind in order to save the world.