Of course, Lex Luthor could also realize his goals by simply pursuing legitimate business interests. The primary argument advanced on a TV Tropes wiki page entitled "Cut Lex Luthor a Check" is that if Lex Luthor devoted just a small fraction of the efforts expended in attempting to kill Superman into simply completing his legitimate business interests, he would be much better off as the straightforward businessman than as a supervillain. For example, one of Luthor's schemes to eliminate Superman as an obstacle to monetary gain actually involved becoming president of the United States. At the point of becoming president of the United States, why would Luthor give a fuck about Superman any longer?
This TV Tropes entry reads,
A villain who constantly fails at beating the heroes never realizes their intellect and hard work might mean they'd get a lot more done if they did an honest day's work; any attempt at going straight is simply a ruse to lull heroes into a false sense of security. This may be more a factor of maintaining the Status Quo, particularly those with a conceit of realism, and it's usually mentioned that the Mad Scientist is mad after all. Sometimes glossed over at a villain's death with "If only he'd used his powers for good, instead of for evil." This is a dying trope as comic book characters became more complex, but was extremely common for many villains decades ago.
Consider, for a moment, the Trope Namer: Lex Luthor. His earliest incarnations were generally focused on using his mad scientist inventions for the sort of schemes typical in the Golden Age and Silver Age, with the goals of pure monetary gain, "ruling the world", or eliminating Superman as an obstacle to monetary gain... the question is then raised as to why he just doesn't sell his amazing inventions legally.
In the Post Crisis world, however, Luthor was recreated as a Corrupt Corporate Executive, already a multi-billionaire captain of industry before even meeting Supes. Now, having far more cash than a man could ever spend in one lifetime, Luthor's only want is power, and while he certainly has a great deal of it already, he wants more... and Superman, he feels, is standing in his way.
The current Luthor is a far cry from a purely Mad Scientist, he thus avoids the trope.
When this is avoided, the turn to the side of good is usually planned well in advance. Heroes may even precipitate it by simply asking "And Then What?"
Sometimes this trope is subverted by villains who start out using their talents for legitimate gain, but who end up becoming villains for one reason or another.
On the other hand, this Luthorian dynamic may be more complex than the trope named after him portrays it. This can also be interpreted as a quality vs. quantity issue. It is true that Lex Luthor could come into a great deal of money through his schemes. But once one progresses above the poverty line, his or her happiness level does not rise with the amount of money acquired. There may be a slight correlation between happiness and money above the poverty line, but it is so slight as to be insignificant. Lex Luthor was already a multi-billionaire before encountering Superman; he is not likely to become a happier person from making more money. Perhaps the only satisfaction he gleans in life comes from fucking over innocent people. Considering how often Luthor ends up in jail, the sum total of personal happiness he enjoys does not seem to have its source in going after Superman. The time he spends in prison only provides him with more time to think long and hard of new plans for offing Superman, not for getting richer.
Lex Luthor strikes me first and foremost as encapsulating the lifestyle of the high-stakes gambler, who gets a thrill out of the risk itself, even when he or she is terrible at gambling. The High-Stakes Gambler should be recognized as a trope, and perhaps the trope under consideration here renamed accordingly. There may be a flaw in my reasoning; I trust my readers to point them out as they find them.