It is known that Manslaughter used his then-latent powers to track the Interloper in the deserts of Siberia. Impressed by the feat, the Interloper taught Manslaughter to develop his psychic abilities. However, the Interloper noticed that the Manslaughter was mentally unstable, so he did not taught him too much, to not create a threat.

Manslaughter had learned enough and began a career as a paid killer, motivated more because of the sport and fun than for reward. The Interloper tried to stop him, but found himself unable to.

Manslaughter was hired by a crime boss whose recent cocaine smuggling operation had been frustrated by the Defenders, a group of costumed superheroes. The crime boss wanted Manslaughter to defeat and kill the Defenders. However, the Defenders defeated Manslaughter and sent him to prison.

Manslaughter was greatly impressed with the power and style of the Defenders, so he escaped from prison, tracked them, and told them he wanted to join the team. The Defenders refused, but Manslaughter then captured all of them. Most of the Defenders escaped from their traps except for Angel, who was trapped in a burning house. Manslaughter offered to rescue Angel only after he was named a Defender. Just then, the Interloper (who was at the trial of one of the Defenders, Moondragon) caught Manslaughter and used his own powers to extinguish the fire.

The Interloper then convinced the Defenders to accept Manslaughter because they would need him to fight Moondragon and the evil entity who has possessed her, the Dragon of the Moon. Manslaughter was essential when defeating Moondragon by projecting madness and demonic feelings of fear in her mind throughout a mind link that she had established with him during a previous adventure.

Moondragon defeated Manslaughter because her power was increased by the alien entity the Beyonder. The Interloper and Defender Valkyrie projected their own immortal life forces versus the Dragon but, the defeat Moondragon too, their power had to go through "mortal instruments". The Interloper asked Manslaughter for help, but Manslaughter initially refused, assuming that the help would kill him. Thus, the Interloper and the Valkyrie used two other Defenders with that plan; meanwhile, the rest of the Defenders except Manslaughter tried to rescue innocents in danger. When they came back, they discovered that Interloper, Valkyrie, Gargoyle, Moondragon and Manslaughter had became statues of ashes and dust, and the Dragon was "gone".

Manslaughter eventually returned through unknown means and has resumed his former career.