Eh. That's a kind of deep ethical question there, and "they didn't have to" really depends on what happened before that scene, now doesn't it? Or are you going to argue that, just based on the final fight of Endgame, that Thanos was the victim?
In the comics, Agatha was at the Salem witch trials - Agatha actually ensured that several of her fellow witches were tried, as a form of test - to Agatha, no witch worthy of the name could be caught and slain in such a way.
She may not be Doctor Doom, but Agatha's not a nice person.