This was a moment a long time coming, and as one would expect, the writers take their time bringing it to fruition. After all, they've been hinting at it for more than two seasons. This is the episode where the Brothers Winchester come to blows as Sam takes one huge step towards the Dark Side. The result is one of the best episodes of the season (and most heart-wrenching, to say the least).
I can't say that I'm particularly pleased with the way that the final swing happened. Sam's choice to drink Ruby's demon blood was initially his own, despite her urging and manipulations, so at least it was another example of his conscious choice to use something evil for something good. (At least, that was and is his stated intention. We all know how the saying goes.)
This latest turn, making the demon blood like a drug and highly addictive, takes away some of that agency. Now Sam is "under the influence". Had he become a bit twisted with the rush of power, and had that power never truly waned, it would have been a logical extension of the corruption of power.