I think a basic sense of human decency and morality should have governed Cash's actions. That sounds a bit blunt, but still.
Cash knew that what Strohmeyer was doing was wrong, or in his words, "out of character," and yet he did the bare minimum (if you can even call it that) to stop it, and the only reason he didn't do more was because he understood that this was not a situation that he wanted to be in. The fact that he understood that what was about to happen was something that he didn't want to be a part of, but didn't even take the time to consider that maybe, just maybe, this was something that Sherrice also did not want to be a part of but was being forced, baffles me completely. It is made clear by David's attempt at making excuses and reasoning with the interviewers that he is just a selfish person, and there has to be a stronger word for that somewhere because that feels like an understatement. Even when it comes to, as Cash would say, "starving people in Panama," the people who don't actively do anything to solve the problem (donating and such) are at least sympathetic and talk about it with other people. David just, didn't care. It's likely that if Jeremy was never arrested, David would have kept his mouth shut for as long as he wanted.
In my head, a serious wrong is an action that causes any kind of harm (emotional, physical, financial, etc) to a person/people, and these are the kinds of wrongs that people are obligated to stop. Of course, huge societal problems like disease and starvation are not something that can be solved so easily, but when someone witnesses right in front of them something serious happening, then they have to do *something.* They don't have to necessarily go out of their way to physically stop it themselves, especially if they think they won't actually have an effect or if it would escalate the situation, but they should be seeking out help from someone, authority figures, police, anyone. Even if the crime is concluded before law enforcement arrives, at least the person who committed it will be actively searched for and arrested so that they don't just walk free and keep committing.
On the opposite end, wrongs that I would deem inherently "harmless" (until it reaches a certain threshold in which they should stop both for themselves and others) are wrongs that don't cause any direct harm to anyone. These kinds of wrongs might actually have a sound reason behind them, and they should be heard out depending on the wrong. for example, cheating on a test. It's pretty easy to assume that maybe the offender just didn't have enough time to study before then, which is reasonable. When it comes to stealing, it depends. If the person is stealing non-essential items (jewelry, etc), then they should probably be stopped, however when it comes to essential goods like food and water, then they probably have a very good reason for needing to steal these things, and without being intrusive, you should do what you can to support their situation so that they won't feel the need to steal at all.
Now we have Good Samaritan laws put in place that make it so that if you witness a crime and you don't report it or do anything about it, you are considered an accomplice to the crime. Actually, now that I think about it, how do these laws work in group situations? For example, the 36 bus incident that happened in Boston in 2000. According to Daniel Auclair's eyewitness account, there were several passengers who witnessed the violence that occured to that child, and despite whispers and nervous looks, nobody did anything. Are all of those people considered accomplices? Also, I fully understand that this law is enacted for entirely good reasons, but at the same time, you can't just ignore the fact that, some situations truly are just too overwhelming for people to act. Going back to the 36 bus, Auclair recognized that something bad was happening and really did consider interfering, but he didn't. I'm sure many of the people on that bus felt the same way, but there are many reasons why a person wouldn't want to interfere. They could be in shock, they could be risking their own life or wellbeing by interfering, they could simply just have no idea how to help in that situation.
Deborah Stone makes a point of this in "The Samaritan's Dilemma." It's not like no one wants to help, it's just that they feel as though they are in a situation in which someone else will surely come along to save the day, not having to put themselves in any danger. Even some of the people who did end up being "good samaritans" had a moment of hesitation before stepping up. Or in other cases, self-preservation was prioritzed, even by police officers. If a person hesitates before helping someone, especially in really dangerous situations, then that's reasonable to an extent, that's just human self-preservation, but I guess there's a difference between hesitation and choosing not to do anything altogether.