In the Overwatch short “Alive”, Tracer is seen barely stopping Widowmaker from assassinating Mondatta during a speech.

In the LE SSERAFIM music video, she shows that she is able to rewind time, and bring objects from the past into the present. It’s unclear how long their drive to the concert was, but one could reasonably assume it was upwards of a half hour. With her Chronal Accelerator, she definitely could have prevented his death.

This shows that Tracer was more willing to not miss the concert than to save Mondatta, and as a result Tracer singlehandedly knocked down the first domino that lead to the current Null Sector invasion.