The McAdam and Milne edition of Dr. Johnson's Dictionary does not offer a definition of "internecine" perhaps due to its abbreviated (465 v. 2600 pages)form. The following note appears in "The Merriam-Webster New Book of Word Histories" (Springfield, 1991, pp. 242-3): "The Latin noun nex 'violent death' gave rise to the verb necare 'to kill' and internecare 'to kill without exception, to massacre'; on the latter the adjective internecinus 'fought to the death, devastating' was formed. In Latin the prefix inter- did not always carry the meaning 'between' ... but was used in some words to denote the completion of an action, which in the case of internecinus means 'to the death'. ... When Samuel Johnson was preparing his Dictionary (1755), however, he was either unaware of the completive meaning of inter- or (more likely) misconstrued its meaning in the available examples of internecine, since he defined it as 'endeavouring mutual destruction'. On the other hand, when Noah Webster came to define internecine for his Dictionary (1828), he gave only the original sense of 'deadly, destructive'. It was not until the 1864 revision of Webster's Dictionary that the Johnsonian sense of 'mutually destructive' was added. This latter sense gained acceptance among the literati of the nineteenth century, superceding the word's original meaning. ...

This sheds some light, yes?