You're assuming people are rational actors dr dre, but they're often not. There's always going to be disputes. Of course passengers should follow instructions, but when they don't the crew should have the skills to de-escalate.
As davidclarke posted just above you, there's a common thread in just about all of the publicity concerning QF Group lately - lack of customer service skills, which comes from lack of training, and perhaps a genuine lack of empathy from someone's personality - in which case they should never be hired.
It's a good thing that many posters here in the thread aren't hostage negotiators. The hostages would be killed every time!