After the relative success of Man of Steel (a film that made good money but didn't seem to really have the backing of critics or fans), DC followed it up with a semi-sequel, semi-crossover, putting in the only character in comics arguably bigger than Superman: the Batman. Sadly, the results were just not good.
The production of Justice League was troubled, with original director Zack Snyder leaving part way in (for family reasons) and Joss Wheddon coming in to do extensive rewrites and re-edits. Although the final film is watchable, it's still not great in comparison to the filmic output of Marvel.
Before we get to the new universe, we have to reboot away the old one. Or, in this case, several old ones as the current "Prime" continuity for this film is really an amalgamation of various versions of DC continuity that's come before. And then Flash goes back in time and ruins everything.
When a movie has a title like this, I think we all know what's coming. Based on the comic series of the same name, this film is only going to end one way, and it won't go well for our titular character.