Its never specified, but judging from William's smile in the Foxy Go Go Go minigame, he loves killing children. I doubt this attack was driven by rage or anger, but rather because William saw a chance to kill her and did it for fun.

Are you talking about Charlie? I think William might have coerced Henry to let him use his animatronics as a way of experimenting on kids, by killing Charlie, so yeah he was probably pretending to be his friend.

No, he's just an insane killer.