Just like Superman and the rest of his supporting characters, there have been numerous takes on his arch foe over the years. Lex Luthor has been cast as a madman, a brilliant-but-twisted scientist, a captain of industry, a civic leader, and even President.
But how'd he get so... evilly?
There are many different versions of his background, but the origin that seems to have retained the most traction with fans flashes back to Luthor's and Superman's shared childhoods.
As the story has it, Superboy and the brilliant Luthor were friends. When Luthor tried to help Superboy by inventing a cure for Kryptonite, there was an accident; he dropped a flask containing chemicals and started a small fire.
Superboy, on his way to see his friend, noticed smoke coming from the laboratory and immediately blew the fire out with his super breath. At the same time, though, his breath caused the Kryptonite cure to go all over Luthor, causing his baldness.
Now baldness could make you mad, but a raving psychopath? Modern comic audiences (and creators) have demanded a bit more sophistication, hence the layerd background that has at various time included abusive parents and other psychological time-bombs.
On the TV series Smallville, the lose of Luthorís hair is seen in in a different light. He is seen as a very young boy in the pilot episode and this version does not blame Clark directly, as Lex loses his hair because a falling meteor lands near him and the force blows him across a field and his hair right out of his head. Which, you know, happens frequently.