I personally thought American Son did more damage to Norman's character than Sins Past with Norman trying to actually murder his son among other things.
Sins Past bothers me because IMO because it's too convoluted and takes away from the simplicity of the original "Death of Gwen Stacy" story. But I don't think it ruins Norman's or Gwen's characters. With Doctor Light, it's just "Let's make him Rapey McRaperape" and that trait was all that defined his character from that point on.