Well... My plot instincts tell me something happened in Yuu's past that released a magic freeing her hatred/anger side or something. So basically a clone is going around killing people and the souls are being used to either power it up or create these giant creatures "Megalos". I don't think yuu has anything to do with them really. Seems the most obvious.
There is another way it could be though that is typical, we find it in blood+. Evil twin sister that doesn't value life whatsoever and thinks humans don't deserve to live, apocalypse, etc etc..., and there was something in the past that involved yuu and her, maybe even a love triangle... I am sure of one thing though, the murderer is not a typical human, the eyes glowing red during the kill gave that away too easily.
Too early to make any serious decisions. I like these kinds of stories that don't give the whole story away at the beginning then play off of it for the remainder of the series/manga. It's a trait not found in many anime/manga anymore IMO.
"The people cannot be [...] always, well informed. The part which is wrong will be discontented, in proportion to [...] the facts they misconceive. If they remain quiet under such misconceptions, it is lethargy, the forerunner of death to public liberty. What country can preserve its liberties, if it's rulers are not warned [...] that this people preserve the spirit of resistance? The tree of liberty must be refreshed from time to time, with the blood of patriots and tyrants."-Thomas Jefferson