Complex sentence: Still mourning over Ayano's death, at 18 he became fed up with Ene and killed her, before slitting his throat with a pair of scissors and coming in contact with the Heat Haze, however exact details are still unknown.
Split into simpler sentences: Still mourning over Ayano's death, at 18 he became fed up with Ene and killed her, only to slit his throat with a pair of scissors. It is known he then comes in contact with the Heat Haze, however exact details are still unknown.