Complex sentence: Her determination goes from being merely obsessive to homicidal when she realizes that she and A-ko were rivals back in kindergarten, and creates a series of killer mecha to defeat A-ko before the morning bell rings.
Split into simpler sentences: Her determination goes from being merely obsessive to homicidal when she realizes that she and A-ko were rivals back in kindergarten. B-ko creates a series of killer mecha to defeat A-ko before the morning bell rings.