Final answer:
When Mortimer's window is thrown open and assassins step inside best supports the interpretation that Mortimer was afraid of Vladimir.
Step-by-step explanation:
The detail from the story that best supports the interpretation that Mortimer was afraid of Vladimir is when Mortimer's window is thrown open and assassins step inside. Mortimer is not present in the room, but the assassins became enraged and wounded each other. This event demonstrates Mortimer's fear and his ability to escape from dangerous situations. This evidence shows that Mortimer's fear of Vladimir is justified.