Bill in the mod won't actually have a voice beyond the gameplay vocals and gameplay shenanigans. There WILL be his audible laugh, so its important you can pull off Bill's laugh.
Just read the lines in the way you'd think Bill would say them.
First line is cocky, as he knows he can't lose and leads the protagonist(BF) into a trap
Second line is him mainly having a mocking tone in response to BF not understanding a certain situation

Good luck :)