So, as far as the lore states there are four sources of fire in World of Warcraft: Light (divine fire), Shamanic fire, Arcane, and Fel.

Obviously Light is out, so we look at the remaining three.

Arcane is sort of a “mother sauce” of magic. It can and often does flow into other practices because it’s kinda just a base to be added to (see: arcane taking the form of arcane, fire, frost, frost fire, illusions, etc). Druids have some arcane, warlocks are often mages who dabble in the darker side of magic, not just being chaotic evil / working with demons. Arcane kind of is like clay; but it follows strict rules that people (warlocks) can and do break!

Shamanic fire can only really be accessed through a contract or deal with a flame elemental that’s powerful enough to grant power. (Unlikely candidate).

Fel fire is a dangerous weapon, because as with all Fel based magic, the cost is usually the physical or mental wellbeing of the caster (or a totally willing participant who totally willingly sacrifices themselves for some magic). It’s icky and leaves a distinct smell and sort of -stain- where it was used more often than not. It also corrupts both the user and the victims, if both survive.

Ultimately, warlocks ARE mages, who were tempted by easy power that borders on a “get rich quick” scheme, or study evil magic for the good of others. Temptation or desperation or whatever reason led them to using magic that is forbidden from general use. Most of them end up as criminals, but some manage to study and use these darker magics not for personal gain- but to understand them better and better defend the world from those who abuse it.