And he knew it because he didn't show any confidence that she'd stand down. He was still on edge after the bells rang because he wasn't sure she was going to back off.

>She looked at Greyworm and said something like “you’ll
>know when I give the sign”
>
>Greyworm was ready to kill everyone. I think both of them were
>in some kill kill kill for Missandei.
>
>Hell, Missandei’s last word was fire.
>
>and a Mad Queen isn’t calculated. The reason they are call
>Mad is because it’s a quick turn from stable to unstable.
>Everyone been saying g these Targaryens can’t be trusted
>because they crazy.

We don't know if she's truly "Mad". It will depend on how she tries to justify herself next week.