People will rob you and take your gun off you if they see your gun before you notice you're getting robbed. If you draw on someone pointing a gun at you you will get killed.
Some states allow you to protect property with deadly force and others do not. You have to read state law to understand the nuance.
In San Francisco you can generally take anything you want from anybody and the victim will generally get in trouble trying to stop you. You may get arrested for the crime but there is no guarantee. The police will release you soon after you are arrested, often on a signature bond with zero deposit [1]. You can rinse and repeat this process dozens of times per year because it is unlikely you will spend any significant time in jail or prison. You'll have a database record but no problem making ends meet upon release.