Karna was standing there stunned not understanding how he could run into this legendary figure. A man who was said to have the power to end the whole war of both sides once angered, Not only that but also the famous trio of friends.
Karna was not sure how to handle this situation now, It was unexpected for him to meet Ashwathama. According to the original plot he was going to meet him when Duryodhana introduced Ashwatthama to him after the tournament to show the powers of the Princes.
So this is very unexpected, Karna falls into conflict about whether to do something or let the plot unfold on its own. Either way, they are going to have a warrior master of all in their camp.
"What…What is your other name?"