He knew about it after the event from his conversation with Lady Calvieri - that was when he threatened with the line "you didn't even know...".

I would have thought that the attempted execution was sufficient to establish their evilness, but maybe it needed the actual act. I also would have thought it was reasonable to suppose that Isabella was in trouble and thus an imminent rescue was in order which was not attempted at all.

But then I am not a script writer, and probably Doctor Who is all the better for it!