From https://en.wikipedia.org/wiki/History_of_union_busting_in_th... --

"Pinkertons and militia at Homestead, 1892 - One of the first union busting agencies was the Pinkerton National Detective Agency, which came to public attention as the result of a shooting war that broke out between strikers and three hundred Pinkerton agents during the Homestead Strike of 1892."

Maybe not "hitmen" as such, but certainly the violent enforcer arm of capital hired to attack striking workers.