Often we consider a just action to be ethical, and ethical actions to be just. It seems reasonable then to suppose then that the principles of justice are some portion of the total principles that make up ethics. I would argue however that while just behavior and ethical behavior often overlap it is not necessarily so, and from this that, while justice and ethics may indeed justify some of the same actions and principles, they are really two different ideals with different principles, that may in fact come into conflict.
In general I would say that ethical principles are motivated by mercy, to act ethically is to act in the best interests of both yourself and of other people. Of course different ethical theories disagree as to what is in the best interests of people (pleasure, lack of pain, achievement, ect), as well as to the extent to which the interests of others should be weighed against your own. In contrast principles of justice seem to be motivated primarily by considerations of fairness. Generally it is accepted that to act justly towards other people it to treat them all equally except when they have merited special treatment by some action of theirs (for example you give merchandise only to the customers who have paid you, not to all of them equally).
To understand how the principles of ethics and justice are different from each other it is perhaps best to examine cases in which they disagree. Consider then a principle under which some money is taken from the richest members of society and used to provide for the poorest members. It is reasonable to suppose that, if the amount of money taken from the richest members was small enough not to inconvenience them greatly, the principle might be ethically sound. However such a principle could not be considered just, since it isn’t fair for the money of one group of people to be taken and given to another group of people, unless it is as some form of punishment imposed on the first group of people.
Likewise principles of justice and principles of ethics disagree as to the purpose and nature of punishment. Viewed by ethical standards punishment (such as imprisonment) is justified because it prevents or discourages the criminal from hurting more people. However ethically there is no reason to make the imprisonment (if that is the punishment that we are considering) especially uncomfortable. In fact prison under a perfectly ethical society might be like a moderately priced hotel, practical and not especially uncomfortable. One would also expect that under such a system great attention would be given towards helping the prisoners resolve their psychological problems and towards aiding their integration into society as productive and happy citizens. Principles of justice however support different kinds of punishment, and for different reasons. Under such principles punishment is not justified because it prevents harm to future victims, but because it is restoring the balance between the criminal and their victims. Thus the punishment itself should inflict as much harm on the criminal as the criminal did to others (although not necessarily in the same form, perhaps crimes of violence can be balanced with a long sentence in an unpleasant facility). Punishment motivated by principles of justice is not performed with special consideration towards the criminal’s future actions. If the criminal chooses to remain a criminal then they will simply be punished again; this is all that justice demands.
So far we have simply examined how various policies might be justified by principle of justice and ethics, however these principles can still pull us in different directions when considering action even in a single case. For example consider Plato’s famous example, where a madman asks for the return of a weapon he has previously lent you. It seems clear that on ethical grounds that you should not return his weapon, out of consideration for his future victims. However in terms of fairness it seems equally reasonable that you should return his weapon. After all he has not wronged you in the past, and thus you have no grounds not to return the weapon.
From these examples then it seems clear that we can’t simply describe justice as part of ethics. So then why do ethics and justice tend to overlap in most cases? Well usually what is just is in everyone’s best interests, and usually what is in our best interests is fair, there is no denying that even if we can provide examples that show that it is not always so. It is probably because of this overlap that justice and ethics are so often confused. For example consider the “ethical” principle that I often work with, that when you take you should give something of greater or equal value in return. It would be more accurate perhaps to call this principle one of justice, not ethics, but even so when it is applied to many situations the results it gives agree with our ethical intuitions.
When justice and ethics conflict then which should we choose? Personally I would lean towards justice, but perhaps this is simply because I have never had enough sympathy for other people. To me it seems that the principles of justice are generally superior since they work primarily with what has happened (in order to achieve a balance between all parties), while the principles of ethics rely more often on predictions of the future (in order to determine what is in people’s best interests), and this makes them somewhat unreliable, as our predictions can often be in error. However I will not pretend to be able to solve this question here, although I will say that you should expect to hear more on it in the future, since it is something I continue to work on.