Who really killed Gideon?

Counter targed spell, then exile it. As long as it remains exiled, you may cast it and you may
spend any type of mana for it.
"Haha, Bolas, you will not win and I will not die!
I've prepared the last spell that will save me. You're
doomed! I'm ready to cast it. Just wait...
Where is my spell!? Noooo!!! Palda!!!!?
What did you do?"
Those were the last words of Gideon Jura