Obligatory comment about people continuing to inexplicably read apparently what they think is the crappiest manga in existence and making sure everyone else knows both of those facts so we can backslap and applaud each other or something. Irony abounds as to such 'keen-eyed' readers continuing to read slop like they're addicted.
With that out of the way, I'm thinking its possible she wanted to commit suicide or something. It's not like she's fading right away, so it seems she still has time to talk before she goes....if she does.
Alternatively...and probably more likely...they never found out her weakness. Just because she's hooked up to a life support machine doesn't mean she can't regenerate like other witches. I think she just regrows her head as would be expected of a witch. She'll remind them of that as explanation of why she didn't try to stop or correct them...maybe to make some other point about witches? Who knows.