You write about common sense but with that same reasoning, common sense would also mean not antagonizing representatives or speaking ill of the late queen and current king of a realm that holds the power to wipe out everyone in the witches realm with or without the support of the demon realm. It was basically a slap in the face to both the late queen and current King Lucius who had previously saved them from suffering that exact fate. Not sure why you would consider that a ridiculous reason but I do know that wars have been fought for less. As for Lucius needing to look for a new heir... My bet would be on the mc escaping if something did happen because they are the weakest realm that can't even defend themselves when they are attacked and less than a handful of guards were shown during the travel from the portal to the meeting hall. However, due to it being a diplomatic mission and having to deal with the extra baggage of his bond mates it was better to return safely. Arianna wouldn't have been so brazen if Lucius was well and there himself so she basically took advantage of the situation to assert herself.
No one is saying Arianne has common sense...

Well, maybe you are with twisted reasoning, but anyway...
Lucius was weakened and Arianne had Ba'el on a leash.
So, yeah, she was feeling a bit more feisty than she should.
Doesn't make Atlas, or the vampires, pussies for not killing her on the spot...
As far as they knew at the time, the witches had 2 things they needed.
Why completely piss them off, or wipe them out, at that exact moment?
Lucius does go full rage mode against Arianne, but only after he meets with Seraphina about the most important thing needed... And he also knows he doesn't need the witches for the other thing...
Lucius would have been looking for a new heir, not because something would have happened to Atlas, but he'd have proven to be a complete dumb ass...

