Sera's motive against Khan

"Tomorrow and Tomorrow and Tomorrow". Sera, revealed to be a time-traveling Romulan, explains that she wants to kill the young Khan Noonien Singh because a computer simulation suggested it as a possible way to keep the United Federation of Planets from forming. Except she originally came to Earth sometime before 1992 when, according to previous canon, the Eugenics Wars were supposed to have happened, and the Temporal Cold War intervened and tied the whole timeline in knots, so she's been here decades longer than planned and has a few screws loose.

Example of:
Motive Rant