I don't see why ainz will go so far as to wage war with Slane theocracy, there aren't enough reasons to prompt such drastic actions except.. maybe just pure evil? Okay we get it shaltear was brainwashed by the Black scripture, but is it their fault? They weren't even there for her if she didn't interfere with their mission. Now ainz want to Annihilate their country.
View all comments
Ainz considers everyone in nazarick a family member, if I had the power I too would fight anyone that hurts my parents or brothers, also shalltear would had leak all the Intel of nazarick if she didn't kill the wearer of the WI, so they are a threat because they have the power to make a servant of nazarick a traitor which is a huge deal. And the fact there is a world item mixed in all that it is too a good incentive to invade and steal every treasure said country has, including the world item.
So in short they disrespected one of ainz kids forcing him to kill it, and they also have a weapon that could threaten nazarick and cool stuff ainz wants, enough reasons to invade and destroy a country.