Doomsday was artificially created by Bertron, a mad scientist with no morals working on Krypton, Superman's home world, though neither he nor his creator were Kryptonian (this was done prior to humanoid life evolving on Krypton). Bertron wanted to create the ultimate lifeform. In order to do this, they sent a baby onto the surface of Krypton, where it would be killed by the harsh environment or vicious creatures that inhabit the planet. Each time, the lifeform's remains were harvested and used again, to create a better, stronger version of the last. In short, the mad scientist was using the method of cloning to accelerate the evolution of the being he was creating. Through decades of this process, the being which would eventually become Doomsday was forced to endure the agony of death, thousands upon thousands of times; the memory of these countless deaths was recorded in his genes and drove it to hate all life. Bertron himself met his death at the hands of his own creation.
The nameless being later to be called Doomsday was artificially created in the distant past on Krypton, long before the humanoid Kryptonian race had gained dominance over the planet. During this time period, Krypton was a violent, hellish world where only the absolute strongest of creatures could survive (at the time, the world's dominant lifeforms were said to be the most dangerous creatures in the universe). Doomsday's creator was a mysterious alien named Bertron, who wished to create the ultimate life-form. To do so, Bertron and his team sent a baby onto the surface of the planet, where it would be instantly killed by the harsh environment or the vicious creatures inhabiting it. Each time, whatever of the lifeform's remains that survived were harvested and used again, to create a better, stronger version than the previous. In short, the mad scientist was using the method of cloning to accelerate the evolution of the being he was creating. Through decades of this process, the being who would eventually become Doomsday was forced to endure the agony of death, thousands upon thousands of times; the memory of these countless deaths was recorded in his genes and drove it to hate all life. Eventually, Doomsday gained the ability to evolve against what killed him in his prior life without the need of Bertron's technology. At this point he proceeded to hunt down and kill all of the lethal creatures that inhabited Krypton. Bertron himself met his death at the hands of his own creation.
Doomsday escaped Krypton and went on a killing spree across several planets. He began with crashlanding on Bylan 5 (245,000 years ago), a planet where Darkseid was about to wed a princess (in order to obtain that planet's chemical deposits for Apokolips' weapons factories). Just as Doomsday and Darkseid were to meet in combat, Darkseid was forced to flee as the battle had caused the planet's atmosphere to become toxic and therefore worthless to Apokolips. Doomsday managed to hitch a ride on an escaping shuttle.
This shuttle eventually crashed on Khundia. The warring Khundian clans united in order to build protective armor for a warrior named Kobald. His goal was to survive long enough to force Doomsday onto a rocket. Once the rocket was in space, Doomsday killed Kobald and the resulting explosion sent him hurling through space.
He would next come across the path of a Green Lantern named Zharan Pel. Doomsday took his power ring and sensing the power of the Guardians of the Universe, made his way to them. Hundreds of Green Lanterns were sent to stop him and were killed. He continued to Oa where a single Guardian sacrificed himself in battle to defeat him. With Doomsday dead, the Guardians turned their attention to rebuilding the Corps.
In reality, the release of massive energies by the Guardian had caused a tear in space through which an injured and unconscious Doomsday fell. Eventually coming to Calaton, he tore that world apart for three years. With only the capital city left, all the members of the royal family combined their life forces into a single energy being, The Radiant. The Radiant killed Doomsday with a huge blast of energy (laying waste to over a fifth of his world in the process). In common Calatonian burial procedures, Doomsday's body was shackled and masked, and due to his status as a murderer, was shot into space. Eventually, he would land on Earth, the force of the impact of his landing driving the casket deep underground. Doomsday, however, was not dead, as any time he would be "killed", he would return, having evolved past whatever defeated him.