Main character is too stupid

When the guy trying to save to girl's life tells a girl that is about to get murdered to stay inside the car in the dark, and honk if something happens, then runs off somewhere, the movie lost all credibility. I think Forrest Gump would know that would be a bad idea. The movie then became a movie about a dumb guy traveling back in time and messing things up because he's lacks common sense. Pretty much figured out the ending half way in after that.