I believe in cruel spirits and holy spirits the same way I believe in cruel people and holy people.
There are however some spirits that are just obssessed with sex, love, anger, competitiveness, tradition, etc, the same way there are humans obsessed with this, this may be morally "neutral" spirits that can be both good or bad.
As one person opens up, I think both good or bad spirits can be attracted to him, I would believe the spirits coming and going would depend on the spiritual make up of the person. this is one of the reasons I would agree with people that say that it is better to practice yoga with a Guru or very enlightened person, because the aura arouhnd them is good and auspicious one that cruel spirits wouldn´t want to come near while good ones might be around and share their fuzzy feelings
I still meditate on my own though, and share myself with spiritual guidance I trust. Lately, Ganesh and Shiva have my back