And in fact it doesn't mean that. "Internecine" has come to mean a struggle within a group, organization or country. It can also mean mutually destructive, fatal or ruinous to both sides. It can also simply mean characterized by bloodshed or carnage. All of these would be quite appropriate to Wild Bill. But quite uncharacteristically, The Free Dictionary throws in a bit of word history. I'll quote it here in its entirety:
Word History: When is a mistake not a mistake? In language at least, the answer to this question is "When everyone adopts it," and on rare occasions, "When it's in the dictionary." The word internecine presents a case in point. Today, it usually has the meaning "relating to internal struggle," but in its first recorded use in English, in 1663, it meant "fought to the death." How it got from one sense to another is an interesting story in the history of English. The Latin source of the word, spelled both internecnus and internecvus, meant "fought to the death, murderous." It is a derivative of the verb necre, "to kill." The prefix inter- was here used not in the usual sense "between, mutual" but rather as an intensifier meaning "all the way, to the death." This piece of knowledge was unknown to Samuel Johnson, however, when he was working on his great dictionary in the 18th century. He included internecine in his dictionary but misunderstood the prefix and defined the word as "endeavoring mutual destruction." Johnson was not taken to task for this error. On the contrary, his dictionary was so popular and considered so authoritative that this error became widely adopted as correct usage. The error was further compounded when internecine acquired the sense "relating to internal struggle." This story thus illustrates how dictionaries are often viewed as providing norms and how the ultimate arbiter in language, even for the dictionary itself, is popular usage.
All right--fair enough. But the question does cross my mind--where do we have another example of this alternate meaning of inter? Where else is it used as an intensifier? I'm skeptical, but willing to be persuaded. If you've got a good example, please let me know.
Meanwhile, I'm sticking with Johnson.