Doomsday is a creature that was intended to be the savior of all Krypton. Created many cycles ago through a collaborative effort between House El and House Zod, the creature proved too violent to be controlled and too powerful to be destroyed. Thus it was sealed deep underground.
At some point in time, House El and House Zod created Doomsday to protect Krypton. Dru-Zod wanted to use Doomsday to defeat Brainiac and save Krypton, but when Adam Strange revealed that it would kill everyone it finds, Seg-El convinces Dru-Zod not to use it.
As confirmed by both Adam Strange and the Priestess of the Cythonnites, the Doomsday beast is a ruthless killing-machine driven fully by unbridled hatred, animalistic rage, and primal instinct - killing anything it comes across. It was described as a weapon built for destruction.