Who used to give Lennie mice? Why did she stop?

From the text:

Lennie sat down on the ground and hung his head dejectedly. “I don’t know where there is no other mouse. I remember a lady used to give ‘em to me—ever’ one she got. But that lady ain’t here.”

George scoffed. “Lady, huh? Don’t even remember who that lady was. That was your own Aunt Clara. An’ she stopped givin’ ‘em to ya. You always killed ‘em.”