I usually think of sin as being a daily kind of thing, Lashon Hara, gossip, Bitul Torah, etc. But I think these areas of major decisions are the more determining areas in which sin is relevant.
So how does one go about making the right life decision?
I suggest trust in God can be helpful in this area. That is to make a decision based on the idea that I really do not know which path is right. There can be a path before a person that he thinks is right but it ends in death. So we can't depend on our own reason and logic in this area. Especially when we think we are doing a mitzvah. The areas in which we think we are doing a mitzvah are almost always the exact areas of our biggest sins.
I have areas I think are my sins. Many time I was convinced I was doing the right thing, and it turned out that I had made a disastrous decision. So I conclude that there is something about the decision making process itself that needs to be corrected in order for a me or any person to live an upright life. It can't be following reason, nor what he thinks the Torah commands. The Satan dresses up in mitzvot and seduces people by calling to them saying, "Come and do a mitzvah."
The areas that I think are my own sins have given me an amazing perspective and insight about the world. That is doing a sin and getting punished in a way that seems like a direct result of the sin or not listening to my parents because I was sure I knew better than them and finding out that they were right all along has given me more insight about the world the nature of objective morality more than any amount of book learning (even Torah learning) could ever give.
Being against the State of Israel I discovered in this way is a terrible sin. Since then I have tried to speak for the peace of Jerusalem and Israel. There was the Satmar Rav, Joel [who was a tzadik] who was against the State of Israel. But because of this kind of reasoning I decided he was wrong. [Later I found out his objections to the State of Israel had no basis in Halacha, but were based on obscure midrashim which have no legal validity.]
I also learned how right my parents were when they were so upset that I decided not to go to university and learn a vocation. But that is not to say they were against learning Torah. Just the opposite. They themselves put me into Hebrew school on Shabat and encouraged my learning and keeping Torah. Rather the idea of going to yeshiva instead of learning a vocation they saw as wrong and time proved they were right. That is they saw I was joining the "frum world" and they knew that that has nothing to do with authentic learning and keeping Torah. Rather it has to do with joining a social club where one thinks he will do well. Ane they convince him to join because they see they cant survive without a working class slave group under them