Harry Potter: Snape's Best Decisions In The Half-Blood Prince https://screenrant.com/harry-potter-snape-best-decisions-half-blood-prince/
By the end of The Half-Blood Prince, Snape's position as a true villain seems assured. He was helping Draco, who was a Death Eater. He killed Dumbledore and took over as Headmaster. He seems, in every way, to have betrayed Dumbledore's trust and let his true colors show. However, in every installment of Harry Potter, all is not as it seems - especially when it comes to Snape.
In this year's adventure, most of Snape's best decisions seem like terrible ones at the time, and it's only over the course of The Deathly Hallows that the audience really understands why Snape behaved the way he did - and why his decisions were better than they seemed at the time.
Before the school year even begins, Snape is seen making the Unbreakable Vow with Narcissa Malfoy, swearing on his life to help Draco in the mission Voldemort has given him. Early on, this seems like a bad choice, or at least, one that points to Snape truly being a Death Eater.
However, this is a phenomenal choice, on several levels. For one thing, this allows Snape to cement his position within the Death Eaters. Neither Bellatrix nor Narcissa are likely to think Snape is anything but loyal after making this promise. However, at the same time, Snape is simply following Dumbledore's orders. He already knows that Draco has been sent to kill the Headmaster, and that he will be the one to actually do it - which means that this promise allows him to potentially gain more access to Draco's plans, and keep everyone happy, while still doing what he intended all along.
When Draco is caught sneaking around on the night of Slughorn's party, Horace himself is willing to just let it go - but Snape ends up taking Draco out of the party. This seems like a fairly straightforward decision, on the surface. Draco is breaking the rules, and he is a Slytherin student, so it is simply in character for Snape to take him away from the party.
However, there's more going on here than just a simple transgression. Snape knows that Draco isn't concerned with the party, and wants to find out what is going on. This is a good move to get into Draco's confidence, and to get information from him - which he could then pass on to Dumbledore. It doesn't work, of course, as Draco is determined to succeed alone, but it was still a solid choice by Snape.
One of Harry's worst decisions over the course of this year is the choice to use an unfamiliar curse handwritten in an old book on Draco. He had no idea what the curse did and quickly found out that it slashed him open severely, creating multiple wounds on the boy. Snape, of course, knew the curse well and stepped in to heal Draco.
Of all the choices that Snape makes over the course of the series, this is possibly the most obviously good one. It's uncomplicated, he simply sees a student who has been badly injured and heals him. Odious as Draco can be, he is still a child and did not deserve to be cut to ribbons by Harry. Of course, given Madame Pomfrey's other healing skills, she would have presumably been able to handle this just as well, but Snape's quick stepping-in is still commendable.
This may seem like a terrible choice - Dumbledore is a beloved old wizard, a father figure to Harry, and a defenseless old man in this moment, and yet Snape kills him. To Harry, and to the audience, it is clear that Dumbledore is pleading with Snape - and presumably, to spare him - so Snape's use of the Killing Curse is shocking.
However, by the end of the series, it's clear that this was an excellent choice. Snape was not only able to prove himself to the Death Eaters, keep them safe, and work behind the scenes to try and keep Harry safe (give him back the Sword of Gryffindor), but he was able to fulfill Dumbledore's wishes. His choice to kill Dumbledore meant that Draco didn't have to - and the child who was tasked with murder was spared the need to follow through. Draco, eventually, becomes somewhat redeemed, and this could not have happened if Snape had chosen differently in this moment.
This scene is a much larger part of the books than the movies, as is most of the Battle of the Astronomy Tower, when more of the Death Eaters enter the school. However, in the Battle, Harry races after Snape, determined to kill him in revenge - but Snape prevents the Death Eaters from killing Harry. He claims that this is because Voldemort wants Harry to himself, which is enough to convince the Death Eaters.
However, fans know that this is actually because Snape is, to the end, protecting Harry out of love for his mother. And of course, the importance of this decision cannot be overstated. If Snape had not protected Harry, and he had died here, Voldemort may have won - and the Wizarding World would never be the same.