Headcanon: After Bendy saw his best friend strapped to a table, amputated and dead, Bendy had lost all faith in his creators. To escape his grief (because he hates feeling grief), Bendy became hellbent on bringing Boris back, while also thinking of a plan to make his creators pay for their cruelty and treachery. This, and added to his loneliness the entire time, changed Bendy. He began to become obsessed with revenge and began to loose all sense of sanity and rational thinking. Finally, he snapped, and just decided to use Satan's will to resurrect his friend. By using a pentagram, Bendy began to gather victims. Curious teenagers and nostalgic employees went into the building over the years....but most never came back out. Once Bendy has gathered enough bodies, he performed Boris's resurrection....and it succeeded. Though not without a price. Boris was back, but was now VERY Frankenstein like, and was forever mute. Bendy was happy to see Boris.....but Boris quickly became worried for Bendy. Bendy was still cheerful, but it was more sinister. And Bendy was visibly obsessed.....obsessed with revenge, and now murder. Boris then knew, Bendy was no longer the Bendy he knew. Bendy was too far gone now.....and now all that Boris can do is watch, and try to calm Bendy a little with hugs and cuddles.