I love warlocks to little bits! I used to have six of them on WrA, but now I'm down to three and a warlock over on Moon Guard as well.
Probably the easiest way (for me) to deal with warlocks (or any class) is to not consider their class when I'm making the background. As I branch out their story and character concept, I'll just plop them into whatever class makes the most sense - or whichever one I want to play!
-- Pailla is an alchemist first and foremost, and only her family history and their use of Shadow (and later, Fel) has had her involved at all, as it was taught to her growing up. Mixing that bit of knowledge with her alchemical experiments, she decided to go into demonology just so that she could use the demons to experiment on.
Her goal is to manufacture some sort of mixture that works against demons with simple herbal components available to most people across Azeroth, as she's frothing-mouth crazy-obsessed with paranoias about the Legion returning. She definitely went into full-on Warlock-mode because of this, not out of any desire for power or the allure of the unknown.
The woman despises all things demon-related, and doesn't consider them as 'people', enslaved by the Legion or otherwise. From a demon's standpoint, she's probably one hell of a monster.
The only demon she consistently summons is her succubus, as Pailla is technically nearly blind. She beats the demon around the ears and forces her to do her bidding in helping her 'see' a situation or navigate through complex areas when she doesn't feel confident in summoning an Eye to view things. Instead, she relies on her stealthed-demon to help her, and the creature is so cowed that she's fearful of what might happen if she lies at this point.
-- Juliet's not a pet-person. She's an investigative journalist who just so happens to have had a warlock for a partner. Her use of spells at all are for self-defense in situations where she's reporting on or 'witnessing' for historical records something going on in a warzone. She's never really gotten much into full-on Fel use or summoned demons; all of her experience is with the Shadow schools.
-- Tynarian is Pailla's son, so he learned everything from her and his father, though she's still trying to curb his use of Fel. She doesn't feel that he should be around that aspect of magic at all, and is furious with his father for teaching him at all. Naturally, her partner-in-crime Haleth is probably not going to help the situation any, as he's just as impish as Tynarian and just as pleased to use his skills to show off, which will likely get the boy's attention and probably some of his want to emulate the man.
-- Ziichi is a moron. Her adoptive father, Salarous, taught her how to use the arcane when she showed a sensitivity for it - though she had little control at all and he had to try to teach her how to not just burn down buildings trying to understand what she was doing. After he went missing and was presumed dead, his black drake -- Asherion -- was left to be her tutor as per his will. Teenaged-dragon influenced by the madness of the Old Gods + unbalanced teenaged girl with anger-issues and addiction issues = DANGER, DANGER WIL ROBINSON.
Naturally it ended in terrible no-good things, and his willful manipulation of her and her efforts in being as irresponsible as humanly possible resulted in her losing everyone she cared for. She gave up the craft later on, as with Ashe gone and Salarous unable to come Alliance-side to teach her, and Haleth involved with Pailla instead, she had no real tutor. Between that and her slowly growing up into a young woman from a bratty teenager, and taking on adult responsibilities, she realized that the power she was tapping into was ultimately addictive and dangerous, that she and the abusive drake did not have ~twu wuv~ and that she needed to be a responsible person to raise the children she and Larrk eventually adopted.
(Not to mention she accidentally got pregnant through her whole crazy up and down addiction arc with Fel and the terrible life-choices she made at the time, so she was punched in the face with reality. Add to that the fact that her own experimenting with demons resulted in her nearly being ripped to shreds when she couldn't control something so powerful? Yeah, she learned fast how wrong she was.)
Now she's just working with reptiles, side-eyes magic longingly, and raises her kids with Larrk as a responsible young woman - while still saying the dumbest !@#$%^- things known to mankind.
... So there are my warlocks who have developed stories on WrA. SORRY FOR THE NOVEL. I like to think they're fairly unusual and aren't the MUAHAHAHA VILLAINY sorts, but rather more real-feeling people of whom the involvement of warlock arts was more a bit of story and a flavour to the personality, rather than the entirety of their identity.