|Art: 'Triple Self Portrait' by Norman Rockwell|
And the children struggled within her, and she said, "If [it be] so, why am I [like] this?" And she went to inquire of the Lord. [Toldot - Genesis 25:22]
Why did the children "struggle inside her"?
How could Yitzchak, our righteous patriarch, have a son whose very nature even in the womb was inclined towards idol worship?
G-d can either make a person's disposition naturally good or naturally bad. But, even if a person has a natural inclination to evil, that does not mean that he is evil per se, for he is given free choice.
Rather, the reason why he was given such an inclination was to rise to the challenge and overcome it. Thus, Eisav was given a natural tendency to evil so that he could excel in the Divine service of "quashing" the evil inclination.
Even though he failed in his task, we can nevertheless learn from Eisav that if a person has strong desires to do something bad, it means that he has been given the special Divine mission of overcoming his inclinations.
Source: Gutnick Chumash: Based on Likutei Sichos of the Lubavitcher Rebbe