Yeah yeah, everyones sooo op. Why are these types of manga so popular, this is getting super boring. Is onesided slaughter really a powerfantasy? Seems downright pitiful. It's not even edgy anymore.
It's a type of wish fulfillment fantasy, so of course it'd be popular. To answer your question, yes, one-sided slaughter can be a part of power fantasy if it's being achieved by the protagonist, but in this case it's arguably not since it's being fulfilled by the entire faction the MC is part of (though I don't even know why you'd be asking this question which seems to be beside your point).
I wouldn't exactly call it pitiful since a lot of people are enjoying it, but it sounds more like you're upset you can't find enjoyment in this type of story. There's no reason it needs to be edgy, at least not this story in particular. Most revenge stories tend to fulfill the edgy trope, but despite the name, Death Mage isn't trying to be.
Most power fantasies tend to have a similar start or setup, so they're easy to spot early on. I get Death Mage wasn't obvious it was one in the beginning, but once the signs started to show, this would've been the best time to stop if you didn't enjoy such stories.