If we're talking about 'Harry Potter and the Half-Blood Prince', the most obvious and devastating death is Albus Dumbledore. He is killed on the Astronomy Tower at the end of the book — the scene is brutal and heartbreaking because it's part of a longer plan gone wrong and because the weight of it hits every character differently. Severus Snape is the one who casts the killing curse, and the fallout of that moment defines the rest of the series.
Beyond Dumbledore, J.K. Rowling also gives us smaller but still important deaths and memories of deaths. Hagrid's giant friend Aragog dies of old age in this book; Hagrid buries him and it's a tender, mournful scene that shows the quieter side of loss. There are also deaths shown in memories and backstory: Tom Riddle (Voldemort) is responsible for murdering his father and paternal grandparents, and he poisons Hepzibah Smith to steal Hufflepuff’s cup — those incidents are revealed through Dumbledore’s collected memories. Merope Gaunt’s death, after giving birth to Tom Riddle, is another tragic piece of his origin story. So the book mixes immediate, on-page deaths like Dumbledore and Aragog with older, revealed murders that deepen the mystery surrounding Voldemort. I still get chills thinking about that Tower scene and how it reshaped the whole saga for me.