Rashi tells us that during Rivka's pregnancy she felt conflicting movements within her womb. When she passed by a place where Torah was studied, she would feel sensations as if the baby wanted to leave her stomach indicating that he was interested in spending his time learning Torah. On the other hand, when she passed by a house of idol worship, she would feel the very same sensations - implying a dramatically different way of life. Unsure of what to expect, Rivka went to the beis medresh of Sheim to find out what Hashem had in store for her. Hashem told Sheim to relay to Rivka that she is expecting twins, one will be righteous and the other wicked. Hence, the conflicting messages she has been receiving, since some of the movements are by one child while some are by the other.
After this inquiry we find no mention of any concern on Rivka's part. It would seem that she was reassured by Sheim's explanation. Rav Wolbe asks (Shiurei Chumash), shouldn't she have had more reason to be concerned, for now she knew that one of her children was going to grow up to be wicked? He explains that originally Rivka thought she was expecting only one child and she was afraid that he was schizophrenic - vacillating between serving Hashem and idol worship. Once she was informed that in reality she was expecting two children, one righteous and one wicked, she was able to come to terms with this knowledge because it is the normal way of the world - a constant struggle between good and evil.
The Mashgiach elaborates, when good and evil are mixed up into one entity, it is very difficult to overcome the evil. Even if one refrains from sinning for one reason or another, in most instances he has merely pushed the desire into his subconscious, only for it to resurface at a later date. When the evil is defined and isolated, one knows his enemy. It is easier to subdue his negative desires because he has a clear picture of what he is fighting. Hashem created the world in such a way that there is always a balance between the forces of good and evil and it is incumbent upon each person to choose good and overcome evil. However, if one is not sure as to what is really good and what is evil, he doesn't know what is expected of him, and he lacks the assertiveness to truly overcome those things he suspects are bad.
Our fight with the yetzer hara is a continuous and difficult battle. The very first step in this life-long struggle is defining the evil so that we have clarity in our mission. If one isn't sure when it is permissible to become angry and when it is forbidden, and in general has a hard time comprehending what is wrong with getting angry, he will have an even harder time overcoming this negative trait. The same holds true on all fronts of the war against the yetzer hara. The best way to defeat the enemy is to study a sefer that deals with the trait one wishes to rectify, thereby gaining the tools to define the enemy and vanquish him.