Although it seems like an act of betrayal, Snape kills Dumbledore simply because the elder asked him to, when the opportunity came. We know — it's absolutely bonkers! However, when you think about it, the plan actually makes sense.

As it turns out, Dumbledore asks Snape of this because he wants to spare Draco from a wounded soul and the wrath of the Dark Lord himself, Voldemort. We know Draco isn't as evil as his family, and since he can't bring himself to kill someone, Dumbledore feared the boy would ultimately die at the hands of evil. So, he set his sights on Snape to follow through with his death.

Chapter 33 of Harry Potter and the Deathly Hallows, titled "The Prince's Tale," reveals the plan was predetermined.

Dumbledore told Snape, "All the same, try. I am concerned less for myself than for accidental victims of whatever schemes might occur to the boy. Ultimately, of course, there is only one thing to be done if we are to save him from Lord Voldemort's wrath," to which Snape replied, "Are you intending to let him kill you?" Dumbledore responded, "Certainly not. You must kill me."

Snape then asked Dumbledore why Draco couldn't kill him, and he answered, "That boy’s soul is not yet so damaged. I would not have it ripped apart on my account." Instead, he chose to sacrifice Snape's soul because he trusted him the most with his life.

"You alone know whether it will harm your soul to help an old man avoid pain and humiliation," Dumbledore told Snape. "I ask this one, great favor of you, Severus, because death is coming for me as surely as the Chudley Cannons will finish bottom of this year's league. I confess I should prefer a quick, painless exit to the protracted and messy affair it will be if, for instance, Greyback is involved — I hear Voldemort has recruited him. Or dear Bellatrix, who likes to play with her food before she eats it."

Ugh, this entire sequence just hits us so hard in the feels.