This came up, indirectly, in another thread. I thought it could use its own topic.
EDIT: Since some take issue with Real-Steel rules and find adaption difficult, I thought I'd explain:
1) This rule is meant to determine your attitude. Safety can't be subconscious. You'll start to slip.
2) If you don't get why this is important in both airsoft and real-steel, then you need to stop playing. Go home.
3) Kill or destroy can be retitled "Kill" (as in, kill in game) or damage. After all, pellets will drive a hole in sheetrock. Not in carpet, though! You could rethink this as "Be aware of what you're pointing your gun at."
4) Be sure of your target means you must be sure of what you're shooting at. Know what it is. A great airsoft example of this is the rule, "DON'T BLINDFIRE." As for the backstop, I'm sure a few twelve year-old kids have put pellets through a piece of paper or cardboard and into mummy's car.
Hope that clarifies things.