Poor Metallo. Has ever so much work gone into a character whose fundamental role is to be threatening enough to slow the hero down a bit right before getting uppercutted into the Atlantic? There's not a lot of gradation when it comes to Superman's foes. You go from godlike beings and criminal masterminds straight down to Names Thug #2 pretty quickly. Sure, those nameless thugs tend to get armed with sonic cannons or whatever, but they are really just the Dorritos of the comicbook universe.
Metallo is the exception to that - he's usually presented as a thug or heavy, someone who can be a viable threat to Superman but lacks the intellect and drive to really be the Top Boss. "I'm sure you're acquainted with my associate, Superman," says Morgan Edge as he presses a button on his desk. A panel slides open, and Metallo sighs and steps out, knowing he has a good 15 minutes of winning before Supes wraps him up like a lead-lined burrito.
Poor guy! What must really sting is that Metallo is uniquely equipped to kill Superman. His heart is made out of kryptonite, after all. And though I question the wisdom of tying your main power source and your primary offensive anti-Superman weapon together, he really should be able to give better than he gets. Part of the reason for this, as the producers of Superman: The Animated Series is that if Metallo ever actually punches his weight, Superman dies and the comic sort of ends.
So our Metallo is the John Corben version. He's a robot/cyborg made from an advanced metal alloy called metal-0 who is powered by a kryptonite heart. He can use other sources of power (radium, uranium, etc) but they don't last as long or provide the zoom that the glowing green rock does. He's strong/tough and so long as his "brain" stays together, functionally immortal. We're skipping the nanite version because Terminator 2 was like 20 years ago and oh god i'm so old
Ahem. Corben is actually the second Metallo to menace Superman. The first version, a thug named George Grant, was the guy who was rebuilt after being discovered dying by Dr. Vale. This Metallo was a hulking thing, nine feet tall with piston-like fists. He didn't run on kryptonite, just conventional radioactive materials. He faced off against Supes many times in Big Blue's early career (so we'll see him in the short form Action Comics title), but was eventually put down. Not by Superman, but by his own gang.
See, George Grant was a bit of a jerk when human and being rebuilt as a 9 foot tall wrecking machine didn't help matters. While early on in his robotic pummeling career George was a great asset to the gang (which would later develop into Intergang once Morgan Edge took over with his even fancier weaponry), it wasn't long until he became a bit of a loose cannon. Where does the 700 pound robotic gangster sit? Anywhere he wants. Something had to be done, and John Corben was the man to do it.
It was Corben who set up the deal with some shadowy representatives of LexCorp. We'll give you the now 'out of the picture' Dr. Vale's "prototype" to study and reverse engineer. In exchange, you make me the next version. Corben didn't mention his terminal cancer as that would weaken his bargaining position. In the end, Lex agreed on the condition that the gang perform a few 'favors' for him and make sure some competitors come to an accidental end or two (thus Rex Morgan falls down the well). Corben gets a new robot body, Lex gets some new tech to play with (parts of which end up informing Cyborg later on), and better yet, if Corben causes half the damage George Grant did, there may be a market for LexCorp-produced robotic soldiers in the future.
So Grant is betrayed and deactivated. He currently sits, alive and aware but unable to move, in the basement of a LexCorp facility, his sanity trickling away.
Lex even sweetens the deal with the addition of the kryptonite heart. Thing is, kryptonite isn't cheap, so if Corben wants to continue to have access to the stuff, he'll need to perform a few more 'good deeds' for Luthor. The way that the heart is set up is that it consumes small amounts of kryptonite, so if you just have a little bit, it'll go away over time. A larger chunk of rock (in honor of The Flophouse's recurring villain, we'll say 7 pounds worth) will have a more stable field and won't decay over the course of normal operation.
Much of Corben's early career as Metallo is doing the work of Lex Luthor. However, after pretty much stumbling into his own hefty supply of kryptonite, he abandons working for Lex and goes into business for himself. This pisses Lex off, of course, and he points out that he took apart the old Metallo to make a new one, what's to stop him from dismantling Corben too for the sake of progress?
This actually is the threat that starts to drive Corben a bit batty. What Lex said is true: what happens if he becomes obsolete? Even his old gang is starting to tote around weird weapons. How long can Metallo stay on the cutting edge?
Our Metallo is therefore obsessed with self-improvement. He takes work to fund his own upgrades, slowly working on building the New Him. He doesn't hate Superman so much as recognize that Superman in his job security. So long as he's flying about, criminals will need guys like Metallo to step in. If Metallo kills Superman, his work dries up. And he needs to work to get the money and tech he needs because deep in Metallo's lair, the half-dead brain of Dr. Vale is humming away in a stolen braincase, insane and dreaming up the next iteration of Metallo. Corben keeps a tight leash on his pet brain, lest it develop an idea designed to supplant him, not supplement him.
What new iterations will Metallo see? Adaptability is key, so Corben should get some neat upgrades out of the brain before Superman rescues it. Of course, allowing the brain of Dr. Vale to build its own body will be a mistake that Superman will quickly come to regret.