Solve This Murder

Jamz was a sociopath. He particularly hated women, but disliked everyone. He took a lot of recreational drugs to give him confidence and to provide a veneer of normality when he socialised. Nevertheless he was still awkward and disliked. He kept an eye on women whom he hated, or found attractive. Some of his victims were picked because he fancied them a bit.
He was a careful planner. He would plan for weeks, months even before a “case”. 
His victims were selected according to the universally used criteria. They had to have no family or good friends, no one who would interfere, ask questions, or demand answers afterwards.
On the notes passed around it always said that the victims had to be people whom no one would miss. He personally chose socially awkward women. Misfits. He particularly hated non binary women, so he chose them as victims so that he could enjoy his job. 
He had improved the equipment that they used. He was autistic and had a fixation on musical instruments so was able to fine tune the traditional equipment used. He had made it hurt more and do more damage. 
The female whom he had picked was a perfect choice. The handlers whom he had sold her to often posed for photos with the victim, grinning ghoulishly. Dark eyes giving away their ill intent and hatred of the victim. Often their smiles were smug, like they were posing with a trophy.
They bought a house with the proceeds with enough for holidays and a nice standard of living. The victim died slumped in a pool of her own bloody vomit from the internal head injuries. Anything that she had wanted to do with her life was unceremoniously dumped in the bin with all her belongings.
A year after that murder, in a moment of drug fueled confidence Jamz threw another victim to a different gang member. He grew to regret that rash decision more than anything else in his small pointless life.