I've been doing some Harry Potter research and I came across the same question. Why did JKR specifically use the word Warlock?

Because we all know that it wasn't a mistake. During the persecution of the Druids the word warlock became synonymous with the black arts, or a sorcerer/conjurer. So, most assumed that warlock is a label that the Druids gave themselves. In reality you are branded a warlock by a counsel of your peers. According to Druid linguistics the word Warlock is considered a insult. In the Druid language the word warlock means traitor (in the worst possible meaning, almost like a curse word) or someone who has betrayed the their way of life and does not deserve to hold the title "wizard." According to many dictionaries the technical term means:
from Old English w[AE]rloga one that breaks faith, the Devil, from w[AE]r faith, troth + -loga (from lEogan to lie); akin to Old English w[AE]r LIE
I don't like to assume anything when it comes to JKR, but I believe that it is safe to say this guy isn't exactly a saint. Most likely he will betray someone if he hasn't already.