She didn't demand Aethas' head. If she wanted him dead she would've incinerated him instantly. She presumed Aethas knew what was going on because he's the head of the Sunreavers and Sunreavers were responsible. She was arresting Sunreavers, rounding them up for questioning and interrogation (zealous players would be the ones engaging in any slaughter, and fortunately thats not really canon), it's called a dragnet. Law Enforcement does this stuff all the time, but if you start opening fire on the cops and get shot dead, it's your own fault. She wanted to know who was responsible for bypassing her wards. The Sunreavers didn't know, legitimately, but why should she believe their word when it's the second time a Sunreaver has done this to her?