First lets state the obvious. Decisions that are made disregarding ethics are bad decisions. It leads to nothing but trouble. Example: Enron, the decision that the CEO and others made regarding the manipulation of financial records is just unethical. They chose to mislead investors no matter the consequences.
Sometimes the decisions we make may affect hundreds, maybe even thousands of people. Decisions we make may only affect our family, so be careful to think about what you are doing. You may have your own ground rules to go by. It may be that your decision is made based upon the thought that it is a good decision if it only hurts a few people or you may think that if it hurts one it is not a good decision. Some other ground rules that you may use are: cost of the decision, who else should I consult before making the decision, am I authorized to make the decision.
These rules are yours to make up as you may. It all depends on your moral, and ethical values. You learn these, as you grow, both personal and professional.
You may know right from wrong but outside influences may sway your decision to make the unethical choice. Take for instance a manager who was forced to make an unethical choice, and if he did not make it he may be fired. To him it was a good decision because he protected his family from becoming homeless and poor. He maintained his status quo and that makes it right. Should he have said no and been fired? This depends on how strong your personal ethics are, and how bad the consequences are of the decision you must make. I guess it comes down to, can you sleep at night with the decision...