I cant believe it got criticised for the lengthy opening Thomas and Martha Wayne scene and then more so for Batman's change of heart as he was about to kill Superman.
The whole thing was wonderfully done and very emotional in my opinion. The film needed that beginning to amplify the emotional impact of the middle scene.
Batman had spent the whole time dehumanising Superman to justify the idea that he was an enemy to humanity and should be killed but in that moment, Superman's humanity becomes hugely apparent.
Batman has been haunted by his parent's death his whole life so it is no wonder that Superman saying "save Martha" would make him take a step back and question what Superman means by it. This time in the flashback, Batman no doubt recognises that it is him who is the criminal and he had fallen too far.
The whole sentiment of the film was about how hard it is to remain being a good person in a world full of evil. It goes back to that quote from Two Face in the previous films:
"You either die a hero, or live long enough to see yourself become the villain.". Batman saw he was becoming the villain in that moment.
This comment I saw for example
"I can tell you that if I was in the midst of trying to kill someone and I found out that their mother had the same name as mine that it would not make me decide against killing them."

Stupid people are gonna stupid. Some people think he just changed his mind because their mothers had the same name, completely missing the point.
Poor Zack, he tried but I guess some people have hearts of stone and brains of cheese.