First off, it's impossible to stick to the comics. The source material is always (I mean always) too big and too vast. You have to trim the story down and alter it...to fit the format of an animated movie. And this movie works. It is an eye opening story about how the Flash wished he could have saved a loved one. And a master villain looking at ways to manipulate that desire the Flash has. The story is incredibly told. It's violent. Very violent. We are talking beheadings, we are talking getting shot in the head and seeing the hole through the skull. This movie is rated PG-13 for a reason. This movie is NOT for little kids. They even use words like a-hole and SOB (in long form) in this movie.
If you are looking for the first Flash movie...look no further...you have found it. And it's amazing.