Complex sentence: Yet, his bitterness increases when his beloved Kyouko is killed by a monster... even though she tries to control the monster herself in an attempt to stop XIG (presumably she believes in Fujimiya's visions, and wishes to destroy mankind to save the Earth).
Split into simpler sentences: Yet, his bitterness increases when his beloved Kyouko is killed by a monster. Even though she tries to control the monster herself in an attempt to stop XIG (presumably she believes in Fujimiya's visions, and wishes to destroy mankind to save the Earth).