"You dare to insult me, mortal? You, the pet project that was gifted your powers by no merit of your own. A child who has not even lived as long as my youngest pet. Give me a reason why I should not just kill you here and now."
Mark did not say anything in response to Loki's question. Instead, he just gave Loki the most empty stare he had ever given anybody in his entire life. Loki asking that question was enough to make Mark know that Loki could not kill him. If Loki could kill him, then Loki wouldn't have needed to ask for a reason to keep him alive. He would have just killed him.