I believe the chronology is this:
1. Osananajimi is in traffic
2. Souta (ML) pushes her out of way
3. Souta gets hit instead
4. Souta forgets about main heroine
5. Main heroine calls it osananajimi killing "her" Souta
@huypro9090 Souta is the high school boy.. I think what's going on his he was injured due to an accident caused by the MC when they were all kids and it caused him to lose his memory or something like that, so when she asked mockingly "Who killed him" I think it's referring to his memory of her or maybe his complete memory from that age which would completely change him as a child.. Just a guess lol so I have no idea.