Yes, but it's wrong. Kings kill(-ed) kings all the time. For instance check out what the Saudis are doing these months.

That makes it a big question, why it's not happening more often then they can reproduce. And it's not like they are writing on their own wikipedia page "hey, I, prince Saudi XYZ don't kill Saddam Hussein's nephew because <reason>". So of course it's some guessing involved.

And I have yet to see a more convincing one than that they have dirt on each other. It makes a lot of sense. And while you might think that getting killed is worse than getting bad press or going to jail, for them it means losing power, and that's all they have.

So let's not get too romantic here and try to discuss the facts. If you have a really good reason they don't kill each other more often, then let's hear it. But morals or an "understanding" can't be it.