Prof Snape and the Unbreakable Vow:
Narcissa Malfoy forces Snape to make the unbreakable vow to save her Draco , in the event that he is not able to fulfil the task given unto him, by Lord Voldermort and the in the event that Draco is unable to execute it off, Snape would complete the task, so as to save her son.
Snape despite all of Bella's suspicions , albeit a tad bit reluctant for a moment , makes the vow to save her son!
Much later into the year, Hagrid accidently blurts out in front of Harry abt he overhearing a conv/a sort of argument between Snape and Prof DD abt Snape not wanting to do it anymore and DD having the final word saying that Snape had agreed to do it and he had to see thru to it!
Towards the end of the year, when the death eaters finally gain access to Hogwarts , with Malfoy's help, Dumbledore weakened terribly by the poision in the cave and also , having coaxed Malfoy to reveal all that was there to his task to him and also to Harry under the cloak , invisible, pleads , Snape ... and Snape ... taking in the scenario, that Draco , with the death eaters , kills Prof DD with the killing curse and flees, from tehre with Draco as well!
My take:
The argument between Prof DD and Prof Snape was abt the Unbreakable Vow that Snape had given into reluctantly , and that was becoming difficult for him to handle and to stand by, because Draco was being particularly stubborn abt not letting Prof Snape know abt it ... and Prof Snape might have confessed that to Prof Dumblebore...who only urged to stick with it.
One could only assume that Draco did not confess to Prof Snape abt his task and Dumblebore only could figure it out... and he might have tipped Snape off abt Draco's intentions... and typical of his nature , forbidden Snape to take any action except to watch Draco from hurting other students in the process...of his task...
On the fateful day of Prof DD's death, Snape must have realized, that he had to obey Dumblebore's orders ( one would assume that Prof DD had told him prior , what he would intend Snape to do in the course of events that were to follow) , and to save not only Draco, but also himself, finish the task given to Draco, that the boy could not see through, and finish it for him..; Kill Prof Dumbledore.
Even if one were to assume that Snape until the fateful moment was not aware of Draco's task and intentions, he at the very moment he entered the room , and taking in the scene , knew what was wrong and what was expected of him! But I personally go with my prior view that he was made aware by Prof Dumbledore himself!
Snape killing Dumbledore :
A triumph as suggested by the look , harry noticed in prof DD's eyes , of not letting Lord Voldermort have the pleasure of the vindication , but let Prof Snape do it and also of having saved Draco from walking the path of being a death eater... and saving his innocent life...as well!
Prof Dumbledore saved three lives, Draco and Snape... by pleading Snape to do it..and guiding Harry to his quest for the other horcruxes and the missing links... Draco's behavior as well , which Harry wanted a explanation for... and he made sure , Harry had heard from the boy Draco himself! And Harry's life too, by making sure he was frozen/stupefied... and was not seen. Although, it is difficult for one to believe that Harry would die at hands of another and not Lord Voldermort , it was likely he could be hurt and Prof Dumbledore made sure he wasnt.. He did his best to keep Harry safe!
Prof Dumbledore in the best way he could had paved the path and showed him the way and also gave him the rightful explanation that Harry demanded,... and saved his life and those of Draco and Snape as well!
My thoughts, that I thought I 'd share.. would love to know your thoughts too! 😊