In the Doctor Who episode "Blink", when Sally Sparrow took TARDIS key from the hand of a weeping angel, she didn't know anything about them. Honestly, it's not exactly in my mind that she was looking at that time or not, but she couldn't actively gaze them as she didn't know about the rules.
The area was also far from any population, so there was no-one looking at the Weeping Angels. Without anyone looking at them, they were free from the "quantum lock" to kill anyone.
Why didn't they kill Sally Sparrow at that time?