One of the most divisive scenes in Batman V Superman is Batman’s “Knightmare”, and Zack Snyder explained a bit of the thinking behind that.
He explained it during the Empire Podcast, and his explanation isn’t surprising to comic fans who understood what the Knightmare was trying to show:
“Well I mean, let’s just say this. I think it’s okay to look at the extended dream sequence as an impressionistic view of a possible future right? So, and that’s not hard to, that’s in the sequence, so I’m not like spoiling anything or making up anything that you didn’t see. So the connection with Flash (the Flash part of that sequence) you can speculate if, whether or not Flash is coming from that reality or another one, but that’s the fun stuff to try and figure out exactly what Flash is saying to Bruce and what he means, because it’s all… we know… so it’s not… we’re not making it up.”
The one surprising thing from that is the idea that Flash could be coming from a different reality. That could explain his heavily-armored appearance that’s even more extreme than the Injustice version of the character.