We are being given a taste of a dark rationalist, who does not give his enemies dramatic deaths where they get to die like heroes, perhaps accomplishing something through it.

See also Fate/zero & Kuritsugu. And note that it's not just Hermione's death that is fast and cruel, it's the troll's too: Harry steps forward with the stone, stuffs it in, releases it to explode the head, and acidifies the brains in less time than it takes to type that.

Actually, there's a great rationality quote for here:

Getting caught up in style and throwing away victory is something for the lower ranks to do. Captains can't even think about doing such a carefree thing. Don't try to be a good guy. It doesn't matter who owes who. From the instant they enter into a war, both sides are evil.