I've been thinking and actually I'd describe Laius as having an obsession then having a desire. Laius just ... wants to be around monsters and learn more about them? He's already doing that though, so he doesn't really need the demon's help. He doesn't really have a goal he's working towards that the demon use to hook him in. The demon showed him that dream of what life could be like and Laius took interest and seemed to like the idea ... but not enough to remember it. His obsession has the foundation of the kind of complexities the demon wants but there is no strong drive behind it pushing him towards anything. He seems pretty content with his life, actually, aside from the whole sister thing. I don't think the demon would be able to make much use of him unless it could trick him into thinking he really needed more. But despite Laius' obsession with monsters he hasn't really ever gotten to the point where he's let that get the better of his judgement and put everyone in danger. So I really doubt the demon could trick him to the point it needs him to be at. Funny enough, however, what has gotten the better of Laius' judgement is his desire to save his sister. Which I doubt is a desire that interests the demon as it is to easy to fulfill and wouldn't lead to more desire to feed off of.
Marcille, on the other hand, has a very clear defined goal that she is working towards and she is willing to go pretty far into the deep end to achieve (i.e. learn forbidden magic that would get her in major life-changing trouble if anyone found out). And it's an intense crazy goal, as you said. Sounds super filling for a demon. Not to mention she's seems like she'd be super easy to manipulate considering she is trusting the demon wholeheartedly without a second thought at this point. The demon has found it's golden egg, unfortunately.
Poor Laius thinks he's currently entering the final boss fight with the mad sorcerer but has no idea about the real final boss he's going to have to face after he's done. If he's lucky he might get a meal in-between.
I can't see the reply after I edited
╥﹏╥ srry
Laius is probably going to have to actually kill it- i mean it's definitely a demon. If he can...his desires are lofty but hers are the kind of intense crazy that Thistle's was.