One night, a trail of blood ends at the feet of two boys- white haired and red eyed, with matching smiles and cheerful laughs.

post-apocalyptic world with zombies

A post-apocalyptic world - after lots of bombing and goodness-knows-what, the church is put in charge of rebuilding things. Modern technologies still exist, but the buildings are Victorian in style.

Radiation from the bombs (a large-scale war, perhaps?) caused people to mutate - the victims can't stand bright light, steady rhythms (e.g. chanting) etc. and they feed on people, which somehow makes them grow stronger. They're labelled a menace to society and are called "demons". Demon hunters are paid to kill them.

characters

Tian

demons

Body structure is an improvement on the body structure of the original creatures', giving them greater speed and flexibility. Due to the modifications to their vocal chords they can only growl or roar, animal noises. Longer limbs, better forward vision. They have a very destructive drive.

They have red eyes, however, somewhat like an albino, and their skins are also similarly sensitive to light. The sensitivity to light causes them to roam about only at night.

Their hearts are brought back as proof of the kill - rock/crystalline objects that are located where the human heart would have been. (currently, no explanation for this.)

Yue, Yun and Yang are among a few improved demons, the results of genetic modification by scientists who wanted to "aid evolution". Unlike normal wild demons they are capable of whatever normal humans are capable of doing plus the added abilities from being of demon blood, and appear to be different only on the outside - all of them appear to be albinos.
Yun and Yang from the later batch are able to manipulate wild demons. Yue, being the first success, doesn't have this ability.

backstory

After the first few demons appeared following the apocalypse event (whatever that was - some huge nuclear bomb?), some scientists took it as a sign that humans were evolving and sought to aid the process. Experiments were carried out in four major laboratories in the hopes of trying to refine the demons' abilities.

Funding for the experiments is still a fuzzy issue - I think it was the church who funded the scientific research. The scientists believed they were making creatures that would control the wild demons and perhaps make the world a safer place. In contrast to the vicious killers that were the demons, the results of the experiments would be the ones to save humanity - hence, "angels".

Wu Yue was the first success - at least, he didn't go on mindless killing rampages - but hadn't much motivation to live. He somehow escaped, lived on the streets for a number of years, then ended up half dead in front of Tian after a bunch of demons attacked him.

The experiments continued after Wu Yue's escape, producing a few more successes including Yang and Yun, two annoying kids who considered Yue their elder brother and try to get him to go back with them. After they broke out from the lab (taking most of the other successes with them), the experiments were discontinued.