Years ago that happened when I lived in SD. A late night knock at the door and stupid me opened it. It turned out the guy was lost on our rural road and stopped for directions. After I'd given him the directions he wanted to shake my hand and I obliged. Then he left. My son just out of the Army looked at me and told me, "Dad he had full control of you!" He did too. That won't happen again.
Biggest mistake was shaking his hand! This custom was begun as a sign of peace or truce to show you have no weapons. When he has ahold of your right hand, you are defenseless unless you happen to be left handed, and you are too close to defend yourself.
Ran into a similar situation, drunk guy at 1:30am looking for somebody. He was at front soor which I don't use, so I came out the side door, and asked what he wanted. I had slipped a jacket on with a .38 in my right hand in the pocket. I did not brandish it, or show it, just kept it in my hand in my pocket. After I told him he was at wrong house and needed to leave he apologized and wanted to shake my hand. I just looked at him, and repeated "you need to leave". He then apoligised again and said he was sorry and he didn't blame me for not shaking his hand. I told him, "nothing personal, but its 1:30am, I don't know you, and I'm not letting go of the revolver in my hand in order to shake yours." I truly believe he was a lost drunk, but If he had any malicious intentions about coming back, I'm sure he thought better of it. Yes, I called police afterwards.
IMO
Don't answer door = nobody is home
Open door and friendly= sucker
Open door, be polite but unhappy, indirectly let them infer they are covered=you picked the wrong house
Open door brandishing weapon = psycho