It's something I've always liked to imagine and wonder about as well, although I think if I ever heard an official answer it would take some of the fun out of it. Armus is one of my favorite TNG bad guys, and not just because he killed Tasha. He was truly evil and pathetic.
I also love his awesome theme music!
From Memory Alpha:
Armus was born as a by-product of a procedure in which a race of "titans" brought out from within themselves all evil and negative attributes that had bound them to destructiveness. The unwanted substance spread and coalesced into a dank and vile second skin.
Yeah, I don't think the Founders brought out from within themselves all the evil and negative attributes they had. They seemed like pricks to me.
Whoever left Armus there was some angelic do-gooder race when they left. I get the impression that when they left the evil/negative substance, they didn't foresee it coming together to form armus, and probably never realized his existence.. because knowing that, and not doing anything about it, would seem pretty callous to me.
Star Trek Encyclopedia says the race that created Armus was native to Vagra II. So that rules out the Founders too, if it's to be believed. Maybe it was the metrons. With their immense power, they could be said to be "titans", and they seem to be pretty do gooder-ish. Well, maybe a little arrogant, but that's about it.
In DS9, there is mention of a "metron consortium
"... according to "Star Trek Star Charts
" the metron consortium is in the beta quadrant. I can't find any information as to what quadrant Vagra II is in, though. Just that it's in the Zed Lapis sector
I guess Vagra II's quadrant would be irrelevent, though, since the original inhabitants obviously don't live on Vagra II anymore.