I think this may be first supervillain-horror movie. It has a great potential that let us know if superman is evil. As you know every superhero movie has an opposite that fight each other, but in here there isn't any so it make little more interesting to watch. If you ask me; instead of horror element it could just use theme that what if superman is evil and let us know much more of it, it could be much better. Also it could be much better if we see child of mood change/swing and emotional ride of his superpower etc., it could have been better.
Review of Brightburn
