Superman, being the last person you'd expect to prove this, is absolutely terrifying in demonstrating why you should Beware the Superman. Props to the movie and George Newbern for making Superman sinister when doing this, instead of his usual good-natured self in the comics.
Look at it this way: Newbern had to go out of his way to use his Sephiroth voice for Superman. Hearing the voice of one of gaming's most iconic villains coming out of THE big blue boy scout means shit is about to go down.
Superman's supposedly final speech to Lois is that someone has to prove that humanity is more than brutal 'justice,' and that if he must give his life to prove it, then so be it. When he decides to show exactly what the Elite cries for when they think they've killed him, he's like an even more horrifying version of Darkseid. Even Doomsday was a Superman restricted to nothing but strength and stamina. Imagine Doomsday with every single one of Superman's powers, no Superman to stop him, and just as much disregard for human life. At least Doomsday was a mindless bioweapon: Superman seemed to actively disdain the Elite for making him think of them as 'people' instead of 'problems'. This comes frighteningly close to the speech the Plutonian gave to a group of supervillains as he casually divulged their secret identities after his massive Face–Heel Turn.
Superman and Manchester Black lifting an entire section of tunnel, trains and all out of the ocean.
Manchester Black delivers some of the best speeches despite his moral. His speech as they were trying to kill Superman was downright awesome and terrifying at once.
Another good example is his ultimatum to the shadowy organizations and criminal elements of the world after hacking communications worldwide after the aforementioned train incident, which is also when he and the rest of the Elite reveal their team's name.
Manchester Black: This is a call-out to the shadow cabinets, petty dictators and all-around tossers of the world. You're on notice. We're not bound by lines on a map or political alliances or government bodies of any kind. We are our own bosses, and we have a very simple job. There are the good guys, namely us and the bad guys, namely anyone who treats anyone else like trash to further their petty aims. We turn bad guys into memories. So mind your manners, lads and lasses or we'll blow your house down. We're the Elite. You asked for us, world. Now you got us.
Superman's last speech, taken straight from the original comics, is still pretty awesome.
Black: You- You think that's it?! It's not over, you poncy twit! If you think I'll just go to jail and rot, you're living in a dream world!
Superman: Good. Dreams save us. Dreams lift us up and transform us into something better. And on my soul, I swear that until my dream of a world where dignity, honor and justice are the reality we all share, I'll never stop fighting. Ever.
The final fight between Superman and Manchester Black. That Black could almost fight evenly with a seemingly gloves-off Superman and still heavily damage him before getting depowered is a Moment of Awesome in and of itself.
Moreover, Black fights for his own ideals to the very end, even knowing and fearing what he was up against. If he didn't turn out to be a Jerkass, Black probably would've been pretty likeable as a character for this reason alone.
Superman repeatedly bitch-slapping Manchester Black (His slaps are practically the equivalent of a full-force punch from a normal human being) is as hilarious as it is awesome.
Against seemingly all reason, Atomic Skull gets one, too. The Elite are trying to make an example of a meta-human on Superman's home turf of Metropolis. Atomic Skull is, by and large, a B-list Superman villain, yet he absolutely demolishes the Elite before Superman shows up. They've all got powers, certainly, but they're chump-change by the standard of most meta-humans that Superman fights. He rightfully calls them the 'interns' to Superman's 'boss.' It's made abundantly clear that if he were put one-on-one with any of them, he'd kill them without breaking a sweat, and they only win through Superman's guidance. The sort of abilities that let them run rampant among human opponents are nothing in the face of an enemy who went toe-to-toe with Superman, which in turn leads to the final fight when Superman shows them what happens when he doesn't hold back.
And this makes Superman's trashing he delivered to the Skull at the start of the movie even more awesome.
Superman trashes a fleet of Bialyian fighter bombers, and destroyed the weapons they were carrying with zero casualties in under a minute.
Superman: It ends today! Ends without a single loss of life!