Of all the TV westerns I grew up watching in the '50s. Have Gun - Will Travel was my favorite , next to Rawhide & Maverick. The opening theme was written by the famous movie composer Bernard Herrmann, and was distinctly different from the others. My dad just thought he was just another gunslinger, and I had never heard or read the word “mercenary”.
Paladin seems to have an ancient meaning - https://en.wikipedia.org/wiki/Paladin.