What was Astarion thinking when you met him?

This might be an incredibly stupid question, but

When you first meet Astarion, he is pretending to stalk an intellect devourer in the bushes. He asks you to kill it. He's doing that to trick you to come closer so he can hold you at knife point, because he saw you walk past his pod in the nautiloid and assumed you had something to do with the abduction.

Why would someone allied with the mindflayers want to kill an intellect devourer? Because he thinks you're with the mindflayers, right?

"Hello police officer. Quickly, I saw a K9 unit over there. If we work together we can take it out."

Or did I misunderstand his plan