Decisions make people upset one way or another. No principle can make everyone happy, so we should strive to make the least amount of people unhappy as we can. Some people say that utilitarianism can be unjust, but if the majority believe it is just, it is just. What you all believe is just is decided by the majority.
We cannot determine the moral worth of any action by its consequences. The utilitarian doctrine has flaws; it ignores individual rights. We cannot achieve the greater good by violating the rights of the minority. Take slavery for example; African slaves had a huge influence on the economy in the South, but they were stripped of their liberty and treated like animals. Utilitarianism also disregards the implications, just because a given action may result in beneficial consequences does not give it any moral worth.
No, utilitarianism is not a just principle, because there are things that are right and wrong in the world. Utilitarianism stands for the proposition that what is true for one person might not be true for another. This is not correct. There are absolute standards of right and wrong, and people need to grapple with what those are.
Utilitarianism is not a just principle because we cannot always know what the consequences of actions will be. The Golden Rule is at the center of many forms of utilitarianism. It states that we should treat others as we would like to be treated. This is not necessarily just because the Platinum Rule now exists and says that we should treat others how THEY want to be treated. This is a more just way of looking at things in my opinion.