I'v found an project that allow some action in SpringBoard so can we create an app to kill all apps running on the background??
Yes you can but you have to use the private framework BackBoardServices you can find the headers of this framework here
BKSTerminateApplicationGroupForReasonAndReportWithDescription(1, 5, 21, @"Kill all the apps");
or if you want to kill only a specific app
BKSTerminateApplicationForReasonAndReportWithDescription(@"com.apple.mobilesafari", 5, 1, @"Kill safari");