Under any other circumstances, the Doctor would have tried to stop the Master from using the screwdriver. He would have felt pity, remorse, forgiveness... but as Valek's scream of pain echoed throughout the underground lair, he could feel nothing but a sense of relief.
He swallowed hard, forcing himself to look away from the crumpled heap that Valek had become as the master gave his simple instruction. The Doctor simply nodded and motioned for him to lead the way - the words thank you stuck somewhere at the back of his throat.