"We attack this random old man!"
"Gotcha! It was a Dragon God in disguise!"
You see how it's a trick? You see the deception?
If you live in a high-crime area and put a shotgun trap behind your door, then you are guilty for the murder of anyone who dies trying to break into your house. Should they have tried to break into your house? No. Should you have killed them? Also no. You're not in the right just because they're in the wrong. It doesn't work that way.
Why is throwing Bahamut at the players knowing they'll pick a fight with him a better solution than just talking to them?