Seth was shit because his overall design was shit.
True, but I also didn't like that he provided no overall story importance as a villain other than filler.
Once you introduce the big bad, the little bad usually ceases to be a relevant antagonist as you know he wasn't pulling the strings. Knowing this ahead of time makes it even worse. I think of the Dark Knight Rises and Bane. Bane was an amazing and imposing villain until you found out that Talia was pulling the strings the whole time. It completely invalidated Bane as he was now just a lesser force (even worse in that he became a mere henchman).