Imagine you are in an art gallery or museum and you are standing too close to a picture. In order to see the big picture more clearly you need to take a step back. People often say "let's take a step back and think about this" when they have a problem they need to think about. Thus, "to look at her was to take a step back" could mean several things in my mind:
1. To look at her made you stop and think about the meaning of life in general.
2. To look at her made you walk backwarks which is counter productive. Therefore it wasn't a good idea to look at her.
3. To look at her makes you think twice about messing with her.