At some level, whether conscious or not, they are satisfying their pride by rejecting God. In their minds they're elevating themselves above God, making a god of their reasoning, their choices, their lifestyles.
Well, that's true of anyone who is an unbeliever. That doesn't really answer the specific question, though. Unbelievers in general reject God out of pride or ignorance, but if someone goes so far as to affirmatively worship Baphomet or Satan, that suggests a way more deliberate spiritual transaction. Presuming they know what they're doing, what exactly do they think they're getting in return for their worship? Fame, fortune, influence? If so, do they really believe some dark spiritual force is granting them success, or are they just playing along with symbols they think are "edgy" without fully grasping what they're participating in? Either way, it's a different level of rebellion than mere pride motivated unbelief.