In the Doctor Who episode "Lets kill Hitler", If
Mel was raised and trained by the Silence to kill the Doctor,
why don't they kill the Doctor the first time they met him in the field ?
Even after crashing into Hitler's office, they only try to kill him after getting shot and
regenerates into River song.
To me it almost seemed that it wasn't their intention to kill the Doctor (based on their behavior) and only after their change did they suddenly get a brainwave that "Hey its my mission to kill the Doctor, lets get started on it now"