I'm not a demonologist by any means, but I've researched a lot of various topics for writing/gming. There's a grimoire called the Ars Goetia that describes various demons and their rankings.
Several of the Demons it describes can be summoned for relatively benign purposes.
But you gotta take that with a grain of salt. A tome by people who want to summon demons, for people who want to summon demons probably isn't going to be as negative on them.
With that said, your conventional demons are mischievous or outright cruel. If a culture wanted to make positive folklore or stories about a Supernatural creature it wouldn't use a demon.
The only good "demon" I can think of is Sun Wukong. And even then, he was coerced into helping the priest.
Tl;dr- Demons are bad because they are classically used almost explicitly as a means for bad things to happen