Immoral Actions?

As soon as I read the Yaakov-Eisav story, I really had one basic question. I know that most people see Yaakov as this angel that does no wrong and Eisav as an evil hunter, but the whole lentil soup-Birthright incident seems a little strange. How is Yaakov able to take advantage of his brother like that? Yaakov sees that Eisav is starving and takes advantage if his weakness. This is not allowed in our religion, and not only that it is also against our morals! Yaakov is the grandson of Avraham, the most moral man that ever lived. The Jews are supposed to be known for Tzedeck and Mishpat, and even Avraham's grandchild can't act with simple morals. Yes, we know that Yaakov was the chosen one, but this really can't be the way that G-d wanted him to get the birthright.

In the passage we read in class by Nahum Sarna, he makes it clear that Yaakov really did have a claim on the birthright, but he was not meant to obtain it in this way. In fact, Yaakov was punished for this action many times throughout his life. For tricking Eisav, Yaakov was later tricked by Lavan when Lavan switched Rachel and Leah at the wedding. Also, to just mention a few of the other misfortunes in his life, his son was sold and his favorite wife died. Nahum Sarna's answer shows me that Yaakov was not in the 'right' when he took advantage of Eisav, even if he was meant to have the birthright anyway, but I would still like to know Yaakov's logic behind his actions. There must have been a reason he had decided to get the birthright in this way. He was an ethical man raised/growing up around the most moral people in the world. He must have known what he was doing wasn't moral, so there must have been a reason why he did it.