Army surgeon and gifted genius Curt Connors had good intentions. The man simply wanted to create a serum to re-grow his amputated arm, a discovery that would benefit millions. He was successful, but the serum mutated him into the lethal Spider-Man enemy, the Lizard.
The two have had plenty of violent back-and-forths over time, with Spidey often needing luck with a side of plot to save him from Lizard's superior strength and durability. Recently in The Gauntlet, the Lizard persona has slapped aside Connor's morals and taken almost complete control. In a tragic scene, the conflicted villain murdered his own son. This new version of the fiend also has the ability to tap into a person's brain, forcing the predator/prey instinct to take over. Lizard abused this power to force chaos on the modern world, but has since fled into the sewers.