I think you're poor example should be more directed properly.
The Elves existed first.
The Elves taught humans magic during the Arthai Wars between Humans/Elves/Trolls of Zul'aman and other northern groups.
Before this the "Guardian of Tristfal" never existed.
We where only able to make it exist after the Elves taught us magic. Leading to our creation of the Kirin Tor and Guardian for protection.
So really it's the Elves.
Or, Wait we should go further. The planet is a Titan Soul. It was cleansed and shaped by other Titans who decided on which races to allow to populate Azeroth.
As well as were aware of the Well of Eternity. Or the likely hood of it's creation, Leading to Elves grasping far more Magic arts or entirely because of it.
So really it's the Titans fault.
Or MAYBE, the fault lays with in the person WHO MADE THE DECISIONS!
Also, it should be said. Medivh was never just a NORMAL Mage/Guardian. He was Sargeras from the MOMENT he was conceived by Agwynn and his Father.
It's not like we trusted someone and they betrayed us.
We were betrayed from the very first moment, there was NOTHING anyone could do.
And Sargeras in controlling Medivh likely did not need the Guardians power to do what Medivh did.
The issue was Sargeras not Medivh, his mom, the Guardians, Kirin Tor. Or whatever. The root is the issue.
Vol'Jin was wounded, Sylvannas made the call she had to made. But that call is EXACTLY what lead to Varians death. Now, Had they stayed could his death have been stopped? Who knows, I'd say probably not considering who he is in the lore and how he acts.
That close to the portal they would have been overwhelmed I think eventually no matter what, even if Vol'Jin had not been wounded. And Varian would have sacrificed himself for everyone. Because that's Lo'Gosh.