The broken down, rusted, and decayed remains of William Afton trapped in a old springlock suit and left to roam through the halls of a cheesy horror attraction based on the stories of his own doing. 

What I'd expect of his voice is something of Scraptrap's but a little more gravely and sounding more like a decayed corpse that still suffers mixed in with an old glitchy animatronic voice box. Adding some coughing and heavy breathing could also add some extra character but isn't required. Also a interesting character quirk could be the angrier he gets the more he voice will glitch.

(Don't be afraid to deviate from the given lines and add your own creative touches just as long as they aren't a complete overhaul of the base lines!)