Thing re Double Negs

From Baugh and Cable, A History of the English Language (2nd Ed. Upper Saddle River, NJ: Prentice Hall, 2002):

For a long time English permitted the use of a double negative. We have now discarded it through a false application of mathematical logic to language; but in Elizabethan times it was felt merely as a stronger negative, as indeed it is today in the instinct of the uneducated. (248)

1) Double negs ok?

2) Instincts. Killer instincts.