I cannot edit a deleted comment, so here is a revision of what I posted:
ISIS did not enact Sharia law when it comes to war. Their unhinged interpretation of the Qu’ran (they did not accept not the full thing ofc just the passages that they deemed relevant for their agenda) was basically that everyone that does not accept their ultra-fanatic extremism is infidel. And then they proclaimed Jihad and in Jihad, infidels are cleansed (also not the meaning of Jihad in the Qu’ran, but ISIS’ interpretation).
It’s best to forget about ISIS when talking about rules of Islam, they are about as much Islamic as the KKK are Protestant Christians.