I actually almost felt bad for the Dalek here. She makes it beg for mercy before killing it because it thinks she’s one of the most accomplished murderers in history.
But, she has no patience for a Dalek who just tried to kill her husband. The husband who doesn’t even know her yet. And really, she has no reason to show it mercy. It just tried to kill him and she’s busy dealing with the fact that if he dies here or erases himself from history like he seems determined to, her entire life will be changed and he will die before he ever knows her. When Amy says he is dead, she’s going on instinct that he probably isn’t dead. That he got out of it because he did. She knows he did, but time can be rewritten and if this goes wrong, her life is forever changed.
Show mercy to this Dalek? Not this day.