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Gr8Pio created a topic of Stairway of Time Webtoon

Ok, could someone spoil me what it's all about? I started reading it and it got rly FKED up rly fast, and I'm not sure if I want to read a story full of toxic drama, I stopped reading around chapter 17, and I have few questions...
Why her bf started to act cruel to her?
Why he left her when she screamed for help while she was locked in burning gym storage?