Uruwashiki Shuen
Complete | Fujitobi | 2000 released
The end's kinda twisted. Person A's world turned beautiful when Person B came. People started eating others n world turned shit and amidst of it B wanted to find his girlfriend and they both came back to the school n saw her as one of those shit and bit B and then A couldn't accept it and cut the bitten area. In the end he get to keep B but B is empty.
2055