I do not use warlocks in my game world, but the very concept of a "pact" with some greater power to "give" you power in exchange for...something...at some point...while being a very valid fantasy archetype (from Faust on down), seems to demand a "dark/evil" power/outlook of desperation.
Interestingly the 4e essentials warlock described it as using a pact to trick power from the infernal beings - so not a supplicant to a power giver, but a potentially someone grey who has found a legal trick to wrest power from the other side and turn it to his causes.
Cheers
I want a warlock that does more than just pay lip service to the idea that you forged a pact with some entity for power.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.