Here's my theory on Snape. Many of these have been posted around the community. I posted this in someone's comment, and got a lot of positive feedback. People seemed to LOVE it. I post this and rest my case. Rest assured I will not be re-posting this 10000000000 of times anymore. And I hope I don't see another 100000000 of them as well.
I still think Snape did it because Dumbledore asked him to beforehand.
1) Because Dumbledore till the end said that he trusted Snape completely. Have faith in Dumbledore, he's not an idiot.
2) There was an argument mentioned between Snape and Dumbledore, where Snape was insisting that he 'WON'T' do 'it', but Dumbledore convinced him.
4) When Harry and Dumbledore got back to Hogsmeade, Dumbledore INSISTED he saw Snape right away, rather than Madame Pomfrey. That suggests that he knew he was dying from the liquid as it is, but wanted to be murdered by Snape.
Why, you may ask? Because he knew that if Snape killed him, Snape would become Voldemort's favourite and closest DE, hence making him the most valuable member of the Order. That way Snape could spy on Voldemort in his closest circles. So if Dumbledore knew he was going to die, he decided he might as well be murdered but by that bring an enormous advantage to the order.
5) When Snape was escaping the grounds, nothing prevented him from taking Harry and apparating with him straight to Voldemort. He could've dragged him out of the gates and have it done, no one was interferring with their battle. But he didn't, did he?
6) The look of hate and disgust on Snape's face before he killed was doubtfully for Dumbledore, but for himself.
7) All throughout the book, Dumbledore keeps saying how his life is less significant than Harry's etc. Hints?
8) In the end, Dumbledore asks Harry to promised to do whatever he would tell him to. 'EVEN LEAVE ME AND RETREAT.'
9) Many think: 'Who would do such horrible thing as Snape did? Even if Dumbledore told Snape so, why would he agree? What kind of person would agree?!' What kind of person, you ask? What about Harry?
Dumbledore made Harry promise to do as he had told him to. Harry force-fed Dumbledore the liquid, even though there was a possibility Dumbledore would die from it. It looked like he was dying. It's called faith in Dumbledore and his desicions. Harry had it, and so, perhaps, did Snape?
10) In the end, before the murder, when Snape bursts into the tower, and he kills Dumbledore. Many say he was fulfilling the 'Unbreakable Vow' he made to Narcissa. But wait a second, the vow was to finish Draco's task IF DRACO HAD FAILED TO DO SO. Draco had not failed quite yet. He didn't give up and say he wasn't going to do it, he was just hesitating. For all we know, some more jeering from the other DE and a command from Snape would've persuaded Draco.
You see, it's more kind of Snape failing to fulfill the vow with killing Dumbledore without pushing Draco to do it. That was Draco's task. By killing Dumbledore, Snape automatically made Draco fail, for which Voldie will now have Draco's head.
I think I got everything and anything. All literary hints from the book.
