HUMAN INSTINCT IS THE "CAUSE" OF "SIN"
Human action is the cause of "sin."
Let's define "sin."
an immoral act considered to be a transgression against divine law.
Sin is not an instinct. We can debate which instincts influences which actions, but we can't necessarily quantify how those tendencies influence a "bad" action anymore than a "good" action.
This changes the syllogism.
THE PERFECT GOD CREATED HUMAN INSTINCT
While this is true, we cannot argue that a Perfect God created the "act."
THE PERFECT GOD KNEW THE CONSEQUENCES OF HUMAN INSTINCT
Omniscience would indeed dictate that.
THE PERFECT GOD FOLLOWED A PERFECT PLAN
I suppose.
AND EVERYTHING IS GOING ACCORDING TO PERFECT GOD'S PERFECTLY PERFECT PLAN
I would suppose that as well.
THEREFORE, WE LIVE IN A PERFECT WORLD
This needs a bit more elaboration, particularly the establishment of the connection between "perfect plan" and "perfect world." Furthermore, I would add that "perfect" isn't necessarily indicative of "good" or "evil" depending on how its defined. It suffices to say that our world "fits perfectly" into God's plan if that's the argument being made. Does this mean that mean that the world itself is "perfect"? We'd have to know that perfect God's perfect definition.