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Deals on Supernatural never end well. John's deal with Azazel to save Dean came at the cost of his life. Dean's deal with the Crossroads Demon (eventually held by Lilith) yielded Sam's life---at the cost of Dean's own. Castiel's deal with Crowley to defeat Raphael lead to the release of the Leviathan. So, it isn't surprising that the deal that the Alpha Vampire made with Dick Roman, leader of the Leviathan, would come with its own catches.
The Winchesters have the translation provided by Tran in hand, and are now trying to make heads or tails of what it might mean. The tools to defeat the Leviathan are in there, staring them in the face. Bobby informs Dean that it will require the blood of three individuals: of a fallen angel, the leader of fallen humanity, and the leader of the fallen beasts. A representative of the three realms, consisting of Heaven, Hell, and Purgatory are found in these. The weapon itself derives from a bone from a righteous human---as light as the Leviathan are dark.
The angel blood, despite Crowley's jab, is the easiest to acquire. Castiel provided it upon Tran's finished translation. The other two are the trickier elements. Crowley could give them his blood now, but he's afraid that the brothers might use it against him---or someone else might. His paranoia about being double crossed plays directly into the theme of this episode.
Double crosses happen throughout---against and in favor for the Winchesters.
While Crowley won't give them his blood just yet, he does give them the location of a safe house for the Alpha Vampire. Upon arrival, Sam and Dean discover a new wrinkle. Three vampires lie dead on a table---heads intact. It would seem that something else has killed them. They discover a false wall and reveal a young woman named Emily, kept for the Alpha Vampire's pure food source.
She sells the brothers a sob story about how she had been kidnapped at the age of eight. It's most likely true, but she's playing it up to convince Sam and Dean to trust her. It works, as she puts on an innocent act---right down to not knowing about Sam's cell phone. Sam and Dean decide to take her with, as she knows about the location of another safe house. They stop at a motel, and put her up there while they go take care of business.
Immediately, she shows her true colors---in front of Bobby no less. She not only knows about cell phones, she makes a call to her "Daddy," none other than the Alpha Vampire himself. She is sending the brothers in as a "gift" and any attempt at surprise will fail for the Winchesters.
Bobby, meanwhile, left behind can't help but feel left out. Out of concern for his temper, Sam and Dean determine that it is best for him to stay at the motel---and Dean locks his flask in the safe. Instead of soothing the flames of Bobby's temper, it ignites it. He is left with little to do---especially once Emily leaves to meet up with her "Daddy." The television is left on, and more news about Dick Roman is broadcast, agitating him further. Despite his ability to control his temper when he is aiding others---mostly---left idle this way gives rise to his anger in a dangerous way.
Bobby never likes to be left behind by his boys---and it has to feel like to him at the very least like a betrayal. He had given them the tip to gather the various blood, after all. Now, they were leaving him behind to execute the plan. Rather than trusting him to go along, they have regulated him to staying behind to sit on his hands. It also adds fuel to the fire---even if the brothers hearts are in the right places. The mere mention of Dick Roman makes Bobby's temper flare to new heights, and each time it takes longer to cool.
Refusing, however, to stand idle, he manages to stoke the fire of his temper enough to do something unthinkable---possess another person and steal his flask. He has decided to join the fight---rather Sam and Dean want him to or not. It is dangerous and troubling as to what might happen to him, once Roman has been defeated---or in the worst case scenario the brothers die trying.