If asking someone who was intent on doing you harm to please leave worked then ya we wouldnt need guns, but the reason that everyone doesnt kill one another isnt because of locked doors or motion lights or home security systems. Criminals dont care if your car is locked. I can break in 90 percent of the cars on the road in under 15 min. Locked doors are no deterrent in a home filled with glass windows. If i am criminal I will just lay and wait and get you to enter the pass code on your home security system.
The real reason people dont go around killing one another is that 1 people are fundamentally good and 2 there is a credible use of force against them.
Criminals want to live too ( most of them ) and look for victims that are not a threat. I dont care if you know where i live or that my doors are unlocked if you know that I dont fear you and I believe me I dont.