Roy felt that this trap devised by Arantir and Astral's group was very… interesting!
When he saw Arantir's undead army being driven out of the fortress through the Demon Eyes, Roy had believed it for a time. If they were really just acting then, then Roy had to say that their acting was the best of the best. Precisely because of this, after Arantir left the fortress with his troops, Roy did not pay much attention to him and focused the investigation of the Demon Eyes on the infection process of the T-Virus.
This allowed Arantir to complete the operation of replacing the original with a fake under his nose.