“...I will not pretend to understand your prerogatives.”
Since his arrival, Dettlaff had been anxious. He was “summoned” by what he understood to be the Grail -- summoned from a point following his death. He could remember clearly his final moments, what had transpired before. He could remember his purpose.
“That” Dettlaff fought for betrayed love, cursing all of humanity, his heart torn asunder by myriad emotions. “This” Dettlaff existed to hunt. To kill. The Grail instructed him to fight.
But nobody fought. And it left the Avenger empty.
“...As a Servant, it is within my nature to kill. Yet none of our kind I have met wishes to meet me in combat. So what of you? Are you, too, a coward who seeks peace where none can exist?”