I think Desmond was just trying to give Locke a "near death" experience.
Why not pursue the love angle like he did with Hurley? Was Desmond confident enough that he could run over someone with a car and only give them a "near death" experience instead of a "full on game over you are dead" experience? What if Locke had been killed?
But I have to remind myself that this show is written by people that have characters think that blowing up atomic bombs will make everything better.
My initial reaction was it was payback for Locke throwing Desmond in the well.