The Nameless Ones by John Connolly

In Amsterdam, four people are butchered in a canal house, their remains arranged around the crucified form of their patriarch, De Jaager: fixer, go-between, and confidante of the assassin named Louis. The men responsible for the murders are Serbian war criminals. They believe they can escape retribution by retreating to their homeland.
They are wrong.

For Louis has come to Europe to hunt them down: five killers to be found and punished before they can vanish into the east.
There is only one problem.
The sixth.