Its so simple.

He obviously killed for the money.

Think about it: Remnant, a manifestation of the soul after one dies in the FNAF verse, potentially could contain a conscience, since the kids were in the suits the remnant likely were in the endoskeletons basically creating some sort of living metal which would make the animatronics more lifelike. Drawing in more customers which means more money, the rumors of haunted animatronics would emerge and the people interested in that would come as well, that means more money. Which Afton can use to fund his experiments on Remnant and potential lawsuits. This also explains how Afton was able to fund the underground bunker, the funtimes, among many others