The thing was that Demiurge came up with a plan, to get Empire to willingly but unknowingly ram a spiky barbed lance up their dry chaffed ass on the requirement of getting a casus belli, and plan was forwarded to Ainz.
Now turn to Ainz vision of the planning.
Say hi to the Empire
Put in word or two about how fucking awesome they are but not else
Wait for bunch of their soldiers to come knock on the door
Jump out "TRICK OR TREAT" to make them shit their pants
Complain to Empire for shit on their front door Attain cleaning bill(casus belli)
Then comes the execution of the plan, Ainz is riding with the investigation party and now knowingly what these dumb stupid fucks are going to do tries to tell them not to cannonball a spear up their ass without fucking up the plan, and gets a yolo worth it response.
TLDR: Workers are dumb fucks, Ainz is a coward, and Demiurge has a massive murderboner.