Mystique's First Kill at the Paris Peace Accords - Late January 1973
Raven Darkholme, aka Mystique, finds the autopsy reports of Angel Salvadore, Azazel, Banshee, and Riptide. She then kills Bolivar Trask at the Paris Peace Accord, before being captured and imprisoned by William Stryker. Interrupting the Paris Peace Accords may of had the consequence of extending US Military Involvement in Vietnam.
Note: Emma Frost's Diamond abilities are used by a Mark X Sentinel later in the movie. In the new timeline, Erik mentions on the plane that Azazel, Emma, and Banshee and their other mutant brothers and sisters had been killed as well (with Mystique, Beast, and Havok seemingly the only exceptions).