(probably because he is like this.)

He wanted to go home. He wanted it so much that he trembled at the thought. But if the price of that was selling good men to the night, if the price was filling those graves, if the price was not fighting with every trick he knew...then it was too high.

It wasn't a decision he was making, he knew that. It happened far below the levels of the brain where decisions were made. It was something built in. There was no universe, anywhere, where a Sam Vimes would give in on this, because if he did then he wouldn't be Sam Vimes any more.

The writing stayed on the silver but it was blurred now because of the tears welling up. They were tears of anger, mostly at himself. There was not a thing that he could do. He hadn't bought a ticket and he hadn't wanted to come, but now he was on the ride and couldn't get off until the end.