Speak up before the train crashes
I'm not good at holding my tongue when I sense that a train wreck might happen. If you've tried speaking up before, and no one listened, it's tempting to want to teach them a lesson.
Okay fine, well the train will crash. THEN they'll see why they should have listened to me all along...
You don't get to take the moral high ground there. If you sense that something might be wrong, speak up before it's too late.
It's your responsibility to convince the others. That means more than saying something once in passing. It means presenting the situation in a way that accurately portrays the severity of what you're seeing so others can see it too.
This is a good video clip from The Last King of Scotland. The disclaimer is that I haven't seen the movie, so I'm only referencing this clip of Forest Whitaker saying: "But you did not persuade me."
Only you know when you've absolutely exhausted your options. Once you've exhausted your options, then you can sleep at night knowing you did everything you could.