If it smells like egg in any way I would have tossed it too. It was probably in the first stages of ruining. Usually ruined chicken smells like rotten eggs.
ETA: No matter what the sell by date is, always pay attention to the store coolers and the smell of the chicken. I bought a chicken once and got home to realize it smelled like rotten eggs. The date on it was well before the sell by date but I took that rotten chicken back to the store and went elsewhere to buy my chicken. Whys was it ruined? Because the coolers needed repair because they were not staying cold enough. If you are in the meat dept pay attention to those mirrors at the top of the coolers. If they are fogged up, they are not as cold as they should be. BTDT lol...it stinks, literally.