The Doctor liked to make his own decisions about people in that regard.

"I would argue there are always chances. I always try to offer mercy, myself. If it's refused, well. Not much to be done for that."

But he makes the offer. Mercy is an important theme for him.

"But you have more experience with these creatures. I can respect your decisions, for now."

Until he's had time to formulate his own opinions about them.