He's talking about Darkseid.

It's talking about Batman.

I don't think he's talking about Clark at all. I think he could be talking about Lex Luthor, but in a way that serves as a dark reflection of Batman. Lex Luthor feels powerless in a world with Superman, and his self-image of futility turns him evil. Bruce is powerless to get over his parent's deaths and his zealous pursuit of justice, and Alfred is warning him to not become like Lex, whether directly or not. Just my theory! :D

It could be either Superman or Batman.

@perchance2dream: Goody idea. I was thinking about the rage and powerlessness Bruce must have felt at his parents death. But now I have a new idea, what if the rage was from all the death and destruction the Kryptonians in MoS caused, and his powerlessness to do anything at that time. And it has made him cruel, in his obsession to protect mankind he is willing to do whatever it takes to stop the alien. Just a thought!

@perchance2dream: My initial reaction was that it was either about Lex or Bruce.

@jbreen: for sure! Superman is a lot of things, but "cruel" is not and will never be one of those things.

Probably Bruce or Lex.

He's talking to Bruce, using Lex as a warning of what Batman should strive to avoid being.

@thebhramabull: he's talking about Batman.

Yeah, he's talking to and about Bruce/Batman in my opinion.

Clearly Batman/Bruce, as soon as he says the word "Cruel" they show the Batsuit.

Clark is far from powerless, he can't be talking about Supes. My guess is he's speaking about Bruce himself, or maybe Lex (Lex was bad before Superman, tho, so maybe not)

He's talking about Bruce.

He talk to Wonder Woman about Batman :P

Yes but they also edited the trailer to show like Clark was distressed by what was happening, people were being nasty to him and he suddenly had an army kneeling before him => good man becoming cruel.

@thebhramabull: interesting point. My money is on Bruce though.

This. It would make sense for either.