Glinda dropped Dorothy's house on the Wicked Witch of the East, not the tornado, and uses Dorothy to gain control of Oz.
One of the first things Glinda tells Dorothy is that SHE killed the witch. They praise her so she'll accept it, and when the Witch of the West comes along, who killed her sister?
Dorothy.
Glinda then puts the ruby slippers on Dorothy's feet but does not tell her that she can use them to go home. Instead she sends Dorothy to Oz in possession of objects that a witch would murder her for.
Dorothy, being forced into a situation where her only salvation is Oz and her worst enemy is the queen, inadvertently exposes the Wizard of Oz as a fraud and murders the Witch of the West.
Now who's left to rule Oz? Glinda fuckin' witch of the North. She used Dorothy as an expendable pawn to gain control of Oz without having to leave her bubble. And when Dorothy is done upheaving the two biggest powers in Oz, Glinda sends her home and makes her think it was all a dream!