You know what, guys? I honestly gotta ask: how the hell is this a bad plot device?
Seriously. It just seems to me that you're just being rubbed wrong by idea that this may turn into a harem manga. Well, too bad, guys. This one's got the "Harem" tag in MangaUpdates. You're just gonna have to either put up and shut up, or go find something else to read. No one is forcing you to read this.
Besides, whether or not this is a bad plot device is strictly a matter of opinion. What truly makes it good or bad is in how it is executed. And we've yet to see how this is going to turn out.
Here's the deal, folks. Yes, this has got the "Harem" tag on MU, so it is very likely that the arsonist's slave is gonna somehow wind up in Harold's possession. How? We don't know yet.
My guess is that he somehow brings this pyromaniac to justice... which means his slave is gonna wind up back on the market as she'll likely be sold for the purposes of restitution. Or maybe she herself will be restitution for the loss of his house. If he could somehow provide evidence that it was the pyromaniac that did it. (Though he'll probably admit to it gleefully in that way that the completely-snapped tend to.)
Either way, I can see Harold not wanting to just cast her off. Not because he'd have a thing for her, but because she's had it pretty bad so far, and he doesn't want things for her to get any worse. He probably won't even be able to set her free because then what? She's got nothing. No home, no money. And even worse, there will probably be a stigma from having been the slave of an arsonist.
And then we're gonna have the oh-so-delightful conflicts between her and Atie. Harold might actually have to force her to behave herself.
So let's wait and see what happens. Unless you read ahead in the raws (chapter 14 part 2 was available since December), you can't really say what's coming up next.