Assassins creed | Why this assassin marked as an enemy? A traitor?

In Assassin's Creed, there is an interesting fact: when you exiting Masyaf, if you activate eagle vision, you can see that one of the assassins is marked as an enemy. I noticed this during my first playthrough, but I still haven't found an answer to why this is the case. What's interesting is that when you try to kill him, the other assassins engage you in combat.
What is it? A developer error? Or an Easter egg?
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