Haha your comment is a puzzle itself :D
So the implicit rule - and I might have not made that clear enough in the tutorial & rules section - is that pieces will murder the king if they are alone with him in one room. So since, as you've indicated, the king must have been alone with a piece in one room at time x, he couldn't have been murdered at a later time y.
You're not the only one that got confused by this, so I'll update the tutorial and rules section to make that more clear. Thanks for checking very closely!