I don't think he chose "Dumbledore" as his password. He was an accomplished double agent, and would have chosen a suitable password that would avoid arousing suspicion from either side.
There are a few potential explanations:
It wasn't the password at all
We know from Order of the Phoenix that the gargoyle controlling access to the Headmaster's office has ...
He's using it as further evidence that Snape had turned against Voldemort and had been playing him the entire time.
He's calling out to Voldemort that not only was his attack on Harry and his family part of his original undoing - but it will ultimately serve to be part of his final undoing and Snape will have had a hand in that, all because ...
From The Half Blood Prince:(emphasis mine)
“I think you next wanted to know,” he pressed on, a little more loudly, for Bellatrix showed
every sign of interrupting, “why I stood between the Dark Lord and the Sorcerer’s Stone. That is
easily answered. He did not know whether he could trust me. He thought, like you, that I had
turned from faithful ...
He's not referring to the commitment to the Order - he's referring to the Patronus, and what it represents.
If the person you love has an animal form, your patronus can turn into something resembling them.
Tonk's patronus became a wolf after she fell in love with Lupin.
As Harry swung the cloak back over himself, she waved her wand; an
J.K. Rowling addressed this in an interview back in 2007. Her answer is short but effective...
Hannah: Why was Snape so badly groomed?
J.K. Rowling: Hmm. Good question. Poor eyesight? Did he look in the mirror and believe he was gorgeous as he was?
J.K. Rowling: I think it more likely that he valued other qualities in himself!
Well you are of course right to suggest in your question that robbing Voldemort of his mortal body isn't a bad plan; he could not be a general, he could not set about acquiring the Elder Wand, he could not lay his plans.
But it would not kill him, it would just leave him bodiless once more. But this time, this time his Death Eaters will not believe him gone,...
Because it wasn't Dumbledore's fault, and because he still loved Lily and wanted to do this in her memory.
No one was expecting Pettigrew's betrayal. Remember that it was Lily and James who made Peter the Keeper a few days before the attack. Dumbledore likely didn't know about this, and even if he did, it seemed a reasonable choice at the moment.
As to why ...
He was invaluable as a spy.
As a double (triple, quadruple) agent, Snape served two purposes:
As an Order agent in Voldemort's ranks, he could pass information about Voldemort's plans to Dumbledore and the Order.
Since Voldemort believed him to be a spy for him in the Order's ranks, he could pass false information about the Order's plans to Voldemort.
In the first summer he met with Lily, Snape used magic to send a branch down onto Petunia's head, hurting her. He clearly had a bit of a mean streak even then.
When Harry accidentally uses Legilimency on Snape, he sees a memory of a young Snape in his room as his parents loudly argue outside. Later in the Pensieve memories, Lily asks Snape if ...
Speculation, but I'm not sure you'll get more than that...
Snape is practical
First, a purely practical reason - he needed Harry to believe him. The emotional connection to Lily is crucial to understanding why Snape and Dumbledore did what they did. Snape can give "what" happened, but to truly convince Harry, he must also give the "why".
Without this it ...
I like the above answer, but my personal fun theory is that Snape refused to use the most well-known hair potion (and the only example I can think of), which was created by....
Fleamont Potter, father of James and grandfather of Harry.
Henry’s son was called Fleamont Potter. Fleamont was so called because it was the dying wish of Henry’s mother that he ...
Snape answers this, and many other questions regarding his DE activities, at the beginning of HBP, when Bellatrix accuses him of disloyalty.
"Do you really think that the Dark Lord has not asked me each and every
one of those questions? And do you really think that, had I not been
able to give satisfactory answers, I would be sitting here talking to
Per J.K. Rowling's interview, "Telling us Professor Snape's patronus and boggart would give too much away", over at Accio Quote:
Ernie: I wonder if you can let us know what form will Professor Snape's Boggart and Patronus take? I am very curious.
JK Rowling: Well, I'm not going to tell you Ernie, but that's because it would give so much away. I ...
They used their nicknames in public.
A gang of chattering girls separated Snape from James, Sirius
and Lupin, and by planting himself in their midst, Harry managed
to keep Snape in sight while straining his ears to catch the voices
of James and his friends.
‘Did you like question ten, Moony?’ asked Sirius as they emerged
into the Entrance Hall.
Snape became a teacher to protect Harry and as penance for Lily's death
Snape felt terribly guilty of his own role in Lily's death. Dumbledore made use of that guilt, and made him promise to protect Harry Potter. Therefore he became a teacher at Hogwarts, because he had to be close to Harry in order to protect him.
“If you loved Lily Evans, if you truly ...
As interesting a question as this is, I think the only way to be 100% sure is to ask her. I don't Twitter, so I'm not sure how this would work, or how likely it is that an answer would be forthcoming, but anything I, or anyone else says, can only be inference.
But, as we know, in Harry Potter and the Deathly Hallows, there is a truly wonderful reveal where ...
If Voldemort had ever known -- or even suspected -- that one of his inner circle was using Occlumency against him, I don't think that person would have lived for very long. Therefore, either:
Voldemort never tried to use Legilimency against Snape (which I find extremely unlikely; especially since, as stated at the beginning of HBP, Snape had to defend his ...
While you are right in saying Dumbledore never told Snape about the Horcruxes, he did tell him when Voldemort would be at his most vulnerable:
"[...]There will come a time—after my death—do not argue, do not interrupt! There will come a time when Lord Voldemort will seem to fear for the life of his snake.”
“For Nagini?” Snape looked astonished.
He did not.
This is based on his conversation with Dumbledore (as seen by Harry in his memories at the end of DH):
“If you don’t mind dying,” said Snape roughly, “why not let Draco do it?”
“That boy’s soul is not yet so damaged,” said Dumbledore. “I would not have it ripped apart on my account.”
“And my soul, Dumbledore? Mine?”
This is a pretty ...
Snape states in Order of the Phoenix that he has been a teacher at Hogwarts for fourteen years.
"Now...how long have you been teaching at Hogwarts?" she asked, her quill poised over her clipboard.
"Fourteen years," Snape replied. His expression was unfathomable. His eyes on Snape, Harry added a few drops to his potion; it hissed menacingly and ...
The Elder Wand is unique in how fickle it is. There are countless times that we see witches and wizards in the books "defeated" by others, and they don't have any trouble using their wands later on (or in some cases, any more trouble than they already had [Neville, after Hermione's Full Body-Bind in SS]). Lockhart and Snape duel each other, and the kids in ...
It's been almost 17 years since Lily died, and about 22 since Snape was rejected by Lily in fifth year after O.W.Ls.
Patronuses change after a great shock, and emotional upheaval (from HP6):
“Tonks’s Patronus has changed its form,” he told him. “Snape
said so anyway. I didn’t know that could happen. Why would your
Lupin took his ...
Snape emphasized its value to get Harry in trouble.
Snape almost certainly didn't personally or emotionally value the Whomping Willow. He certainly wouldn't have positive associations with the place he was nearly attacked by a werewolf.
However, there are two things to consider. The first is, as a professor, Snape would have some idea of the "objective" ...
The Language of Flowers is a means of communication where each flower has a meaning (or several), and the way they are arranged forms a message. It was a bit of a fad in the 1800s but is now mostly unknown except by artists who use it to hide messages in their works.
There are a lot of different messages you can get from the combination of asphodel (a type ...
There's probably a whole host of reasons:
Harry is a particularly curious and observant student
At this point he had already managed to retrieve the Philosopher's stone and get into the Chamber of Secrets before his voice had even broken. Plus he became the youngest Seeker in a generation. If anyone is going to spot Sirius (especially considering he is ...
Horcruxes were a fairly obscure topic, even among Dark wizards.
Though Snape had an interest in the Dark Arts, Horcruxes aren’t a common topic in Dark Arts. When Hermione searches the books in the restricted section, none of them give details on Horcruxes, and only one of them even mentioned Horcruxes.
“I haven’t found one single explanation of what ...
I think something that is not expressed in Gallifreyan's excellent answer has to do with Snape's relationship with Voldemort.
Snape's loyalty to Voldemort was never as deep as his loyalty to Lily. Since Snape was always primarily on Lily's side, Voldemort's ambitions to harm Lily drew a line in the sand that Snape could not cross. He would certainly never ...
He knew that he would have a new job anyway within the year.
Namely, Headmaster of Hogwarts.
This explanation holds for the year that Snape was actually appointed DADA Professor. Why he wanted the post before then is explained further down.
The summer before his appointment as Defence Against the Dark Arts teacher Snape discovered that the current ...
In the first chapter of the Deathly Hallows, Snape reveals the date of the move to Voldemort in a meeting of Death Eaters.
"My Lord, the Order of the Phoenix intends to move Harry Potter from
his current place of safety on Saturday next, at nightfall."
The interest around the table sharpened palpably. Some stiffened,
others fidgeted, all gazing ...