Well, shit.
Time to run, bro! Don't wait, don't check the mail, don't stop to wind the cat, just go!
Grab a go-bag if you thought to have one, but otherwise... git!
Honestly this so called Demon King seemed reasonable enough and especially with his sense of duty. You couldn't work something out? Bro flat out told you his weakness, told you he was dedicated to his duty etc.
"Bro. Me too! Look what they sent to fight you... how about we team up to defend the innocent and fuck over the people who screwed us both"