Professor Diminutio

Professor Diminutio was an early era Marvel comics super-villain originating from Astonishing #6 in 1951 - he was an enemy of an early-era superhero known as Marvel Boy and was an alien who disguised himself on Earth with the sole goal of killing as many humans as possible.

History
Diminutio was an alien who learned how to create a special fabric that would shrink upon contact with human skin, which he quickly decided to utilize in a plan to wipe out the entire human race : pretending to be a tailor he would sell clothing to unsuspecting human victims only for them to be strangled to death as the fabric shrank around their necks.

Eventually Diminutio's evil ways was caught onto by Marvel Boy who confronted the alien, Diminutio protested his innocence and tried to trick Marvel Boy into wearing one of the shrinking suits to prove said innocence (and kill the hero in the process) : Marvel Boy had suspected foul play beforehand and attached a hidden steel collar to prevent himself being strangled but played along, pretending to be dead so the murderous alien would led himself into a trap, where police were informed of the crimes.

Personality
Diminutio was xenophobic and cruel, seeking to kill humanity and seeming to be very inventive in his genocide - mixing elements of a serial-killer with his mass-murdering ways : since he had a very specific pattern in how he killed (always using suits to strangle victims), his victims were all men in the story but he had plans to spread his murder across the world and would of eventually moved onto women and children had Marvel Boy not defeated him.

Like many habitual killers Diminutio was a scheming and manipulative sociopath who could feign innocence when he wanted to lure someone into a trap - such as when he tried to convince Marvel Boy he was innocent, he even planned on stealing Marvel Boy's ship and tossing his body into space after he thought the hero was dead : showing he was not insane and knew very much what he was doing and how to avoid punishment (though like many villains he was not good at checking to make sure said hero was *actually* dead, arrogance which ultimately led to his defeat and imprisonment).