If you kill the nominated enemy, the "evil one" as the American President has repeatedly called him, you no longer have any excuse to create a smokescreen. Without the smokescreen, you can't carry out your hidden agenda/s. Keep the enemy alive, and you can occasionally break a story about thwarted "terrorism" and keep the public nervous, so they will think any steps taken supposedly to maintain "security" are good steps. It won't be too long before anyone who doesn't support populist ideas will be deemed to therefore be the enemy.