When did the word bully pivot?
Solution 1:
The Compact Edition of the Oxford English Dictionary (1971) has an extensive discussion of the development of bully in seemingly opposing senses:
Bully, sb. ... {Etymology obscure: possibly ad. Du. boel 'lover (of either sex)', also 'brother' [citation omitted] cf. MHG. buole, mod Ger. buhle 'lover', earlier also 'friend, kinsman'. Bailey 1721 has boolie 'beloved' as an 'old word'. Bully can hardly be identical with Sc. BILLIE, brother, but the dial. sense 2 seems to have been influenced by that word. There does not appear to be sufficient reason for supposing that the senses under branch II ['blustering gallant,' etc.] are of distinct etymology: the sense of 'hired ruffian' may be a development of that 'fine fellow, gallant' (cf. bravo); or the notion of 'lover' may have given rise to that of 'protector of a prostitute', and this to the more general sense. In the popular etymological consciousness the word is perhaps now associated with BULL sb. [in the sense of male bovine animal]}
The 1971 OED lists the following order of emergence of historical meanings of bully (with date of first cited occurrence in parentheses after each):
[Branch I] 1. A term of endearment and familiarity, orig. applied to either sex: sweetheart, darling. Later applied to men only, implying friendly admiration: , good friend, fine fellow, 'gallant'. [1531]
b. attrib. as in bully-boy. [1609]
2. dial. Brother, companion, 'mate'. [1825]
[Branch II] 3. A blustering 'gallant'; a bravo, hector, or 'swash-buckler'; now, esp. a tyrannical coward who makes himself a terror of the weak. [1688]
b. A ruffian hired for purposes of violence or intimidation. [1730]
4. spec. a. The 'gallant' or protector of a prostitute; one who lives by protecting prostitutes. [1706]
5. attrib. and comb. as bully-critic, -fop, -killer, -rake, -royster, -ruffian, -swordsman; ... [1726]
So by OED's account, what you call the "pivot" and what it refers to as "branch II" had emerged by 1688, when it appeared in Thomas Shadwell, Bury-Fair:
Gertrude. Well, I am of the opinion, that a Lady is no more to be accounted a Beauty, till she has kill'd her Man; than the Bullies think one a fine Gentleman, till he has kill'd his.
It seems quite possible (as OED indicates) that the evolution of bully away from its original sense of endearment or affection toward a sense of reprehensible intimidation and violence owes much to a false etymological connection in the popular mind of the word bully to bull in the sense of a male bovine animal.