Free Will is a sign if God's inescapable weakness

[Edwardo Piet]

I think you misunderstand the purpose of free will in the religious narrative, OP.  It's not there so that we can learn something (some would go so far as to tell you it's not there at all, in the narrative - for another time).  It;s there to circumscribe the borders of moral culpability for obeisance to social norms purported to derive from divine authority.  It exists solely to provide a rationalization for righteousness of divine condemnation....but, much more commonly, for the very real and very earthly punishments man seeks to dole out in god's stead.  Ancient religions, or modern secular legal systems - it's purpose is the same.  A useful fiction.

It's existence or lack thereof says nothing about gods, it really never had anything to do with gods in the first place.  Supposing we find a demonstration of the complete vacuity of the concept tomorrow (this is assuming we don't have it today for no reason other than discussion), the religious will simply go back to claiming that we do not have free will.  That their scriptures always knew this.  That all is and all will be as god wills, and our only true choice in any matter is to obey...and, perhaps...we don't even have a choice in that.

I'm actually of the opinion that belief in contra-causal free will overall causes more hatred and revenge fuelled violence and killing than the most extreme behavior of all religions combined.

Unless you count the fact that it may have itself came from religion. I think it might not have done though, actually I think in all likelihood it predates religion by a long way. When we don't know the true causes of human behavior and we're aware of our own general abilities it's only natural to assume that we are self-causing agents. It's just a lack of analysis. Another example of our so-called "common sense" failing us.

And it may seem benign at first, but it's used to justify not only hatred as a whole but also the most grotesquely barbaric behavior. It also justifies arrogance. When people are full of themselves it's often because they take ultimate credit when no ultimate credit is due. The dunning kruger effect, etc. We credit both ourselves and others far too much, rather than focusing on important things like deterring and mitigating harm.