When I first watched the movie, I couldn't help but be impressed by his evil plans. In every plan, if you notice, he made sure not to let anyone know that he was behind them so that if one of them failed, no one would know it was him. Very clever.

Even after the stampede, Scar didn't kill Simba himself. He sent the hyenas instead because he didn't want to risk being seen by anyone just when he was about to become king.
His first plan involved killing just Simba but in the stampede, he wanted both Mufasa and Simba to be killed. Maybe he was getting reallly frustrated with waiting, or maybe he planned to kill Mufasa later after the hyenas dealt with Simba and Nala in the Elephant Graveyard.
