Killebrew was a scientist who ran a hospice that was supposed to take care of failed experiments from the Weapon X program until they died. Along with his assistant, the Attending, Killebrew constantly experimented, and thereby tormented, countless people who had been deemed failures and sent to his hospice. Many of his own experiments would prove fatal to his subjects. Wade Wilson, the man who would become Deadpool, was one of his favorite subjects, as he was considerably difficult to kill. Fed up with Deadpool's disrespect, the Attending managed to arrange it so that Killebrew would have to kill Wade for disobedience. However, Wade's healing factor finally fully activated when he nearly died, and so he was able to kill the Attending and escape the hospice with his fellow prisoners. Killebrew however, managed to escape the carnage.
Killebrew would later work at a Weapon X facility where he was supposed to help a dying Black Tom, but before Killebrew could begin, Cain Marko (the Juggernaut) came to rescue his friend Tom, and ended up taking Killebrew with him as well so that he could continue to find a way to help him.
When Deadpool helped Banshee and Siryn find Black Tom he therefore also ended up finding Killebrew. At first, Deadpool wanted nothing but to take Killebrew's life, but eventually he just decided to put as much distance as he could between the two of them.
Killebrew would later meet Deadpool again when his healing factor was beginning to degenerate. Aware that Deadpool needed help, Killebrew allowed Deadpool to find him and eventually used some of the Hulk's radiated blood to restabilize Deadpool's healing factor. Despite saving him, Deadpool still intended to kill Killebrew for all he had done to him. The only reason Killebrew lived this time was because Siryn managed to convince Deadpool to be a better man and allow Killebrew to live.
Although Deadpool swore to kill Dr. Killebrew if they ever met again, fate would have it that they did, in fact, meet again. This time they met at the whim of yet another blast from the past. The Attending (resurrected through cybernetics and now operating under the codename Ajax) went on a hunting spree for Deadpool, and killed many of the former prisoners of the Weapon X facility in order to find him. He finally came upon Dr. Killebrew and, after torturing him, was able to learn Deadpool's whereabouts.
After regaining consciousness, Killebrew found himself witnessing a deadly battle between the two titans. The doctor finally decided to take matters into his own hands when he saw that Deadpool, unarmed and dreadfully outclassed, was losing the battle by a long shot. After burning off Ajax's helmet to give Deadpool the opening he needed, Killebrew met his end at Ajax's hands.