The strange thing is John Wayne shot first all the time. If there is guy pointing a gun at you, then there is the specific intention that they are going to kill you. So in that case you shoot first. And the Duke did sometimes shoot first if you had a gun him.
Only thing the Duke had a problem with was shooting a man in the back which is totally different
Some examples I could find:
https://m.youtube.com/watch?v=qzkRpvY9FKA
https://m.youtube.com/watch?v=EfegGAcZwRE