If a person witnesses an injustice, say, someone beating another person, and they don't act, they are morally guilty because they have to live with the knowledge that they could have done something to stop it. They chose not to do it, however, and must feel guilt about a situation that they could have improved.
...Or so to speak. If you have the ability to stop an injustice, then you ought to stop it. If you are willing to see it continue, then you have no right to call it an injustice. Society requires cooperation and contribution in order to function properly, and to refuse to contribute when it suits you is selfishness.
In the face of injustice, there always needs to be action. There is a reason that people feel ashamed if they are caught doing nothing when someone is in need. This is because there is an ethical imperative in society for the strong to help the weak, for action to be taken.
I have heard of case studies where crimes were commit ed on major traffic streets in America with several onlookers, but they all passed by as if nothing was happening. Rather than involving themselves, they act like it didn't happen. If that were a friend or family of mine being hurt, I would want them to get involved.
I feel as if an Individual acting alone against an injustice of any degree is fruitless. Across history, the greatest visionaries who stood up to unjust governments were backed by many followers. In essence, acting against injustice effectively is a societal act, and each individual part ought not to be culpable for the welfare of the society they live in as well as the effectiveness to which it battles injustice.
For example: not every American fought in the revolutionary war, or any war for that matter, so are those who decided to stay home and tend to their personal affairs culpable for the lives lost, or the extent to which a war lasts? No, because overall, the "United States" would be culpable because it is their war. It's societies war.
Ergo, the continuation of an injustice which could be easily mitigated by any affirmative action from a group or community, does not fall on a single individual regardless of his means to step in. If you witness a mugging, and do nothing, you allow one mugger, as well as yourself, go unscathed. If a society is conscience of mugging being a problem in an area, a reliance on vigilante justice will not solve for this continuation of injustice, the only means to the end is to increase Law enforcement patrols, or perhaps increase the amount of lighting on city street. Both of those methods not only solve for this injustice in the long term, but it also protects individuals who would be barred by the affirmative action of this resolution: Act, or forever be responsible, when in reality: the only effective action is done by society, not the individual.
Inaction in the face of injustice makes Society morally culpable
No, inaction in the face of injustice is not immoral, if the person does not have a duty to act. We all have the right to our freedom of person. If someone wants to continue to live their own life and look the other way, rather than step in, that is their right.
I do not believe inaction in the face of injustice is immoral. I believe people witness injustices on a daily basis and if they were to take action on all of these injustices, they wouldn't have time for anything else in life. I think it is important to stand up against injustices but it is impossible for a person to do it all alone. It takes many people.