I don't think he's evil. I think he's Death.
I don't buy into the Link is Dead theory but I think the Mask Salesman and the Moon Children are Grim Reapers of a sort.
That means they aren't good or evil. They're mostly indifferent. The Mask Salesman would be drawn to Termina because the world is about to end without the intervention of Link (and his time traveling Ocarina). He would pop up during a big battle between good and evil (at the beginning of SS) because the body count would be massive on both sides in that.
I don't buy into the Link is Dead theory but I think the Mask Salesman and the Moon Children are Grim Reapers of a sort.
That means they aren't good or evil. They're mostly indifferent. The Mask Salesman would be drawn to Termina because the world is about to end without the intervention of Link (and his time traveling Ocarina). He would pop up during a big battle between good and evil (at the beginning of SS) because the body count would be massive on both sides in that.