After losing the blessing, what did Esau plan to do once Isaac died?

Answer: Kill his brother Jacob

Esau held a grudge and said in his heart that after the days of mourning for his father he would kill Jacob.

“Esau held a grudge against Jacob because of the blessing his father had given him. And Esau said in his heart, “The days of mourning for my father are at hand; then I will kill my brother Jacob.””— Genesis 27:41 (BSB)