Singer must hate Sinister, because he took pieces of him and misused them throughout this franchise.
Knowing they were going to eventually make Magneto a hero, they should have been hinting at Sinister from the get go. Nazi Sebastian Shaw would have been a perfect angle for Sinister, especially since it made zero sense for Shaw's character.
Over 3-4 movies his role gets bigger. He peaks as the main villain in X-III, w/ Dark Phoenix and becomes more complicated, in a good way, in X-IV as they fight Apocalypse.