Facebook broadcast a live video of 3 men in Sweden raping a semi-conscious woman. Only after a long time did the onlookers realize it was not a bad joke, and someone informed the police. The video kept streaming the rape, until the police entered the appartment and arrested the 3 men.
I read an AMA from some Facebook mod once, and apparently, it's just really terrible. You're constantly exposed to all kinds of heinous stuff, child abuse content, rape, and similar to this, people actually broadcasting crimes in progress. Plus, when you're trying to deal with this, you're also flooded with a nonstop torrent of frivolous, bogus complaints and people getting offended about literally nothing, making it hard to find the actual legitimate criminal content.
I'd imagine this was simply a case where someone prioritized actually contacting the police to stop the crime over stopping the stream. Maybe they assumed someone else was doing it. Maybe they realized that shutting down the stream would alert the criminals that their actions were being monitored.
I seriously doubt anyone said "let's just leave this rape stream up for the pure hell of it."