Macrinus was an excellent administrator and the first equestrian emperor. He reigned for one year until the legions supported Elagabalus. Mostly because macrinus was being a realist: he ended the war with Parthia, restored the admenian king to his thrown, and refused to continue paying the legions the massive pay raise Caracalla gave them.
So what if macrinus had killed of Elagabalus and Alexander Severus and the rest of the several family when he became emperor like he should have? Elagabalus was only claimed emperor because Julia misa played it off that Elagabalus was Severus's son. If there's no "legitimate" successor to the severance dynasty, macrinus position is secure. Only the eastern Syrian legions had supported Elagabalus to begin with, the western legions and the senate remained loyal to macrinus until he was dead.
So what if macrinus had killed of Elagabalus and Alexander Severus and the rest of the several family when he became emperor like he should have? Elagabalus was only claimed emperor because Julia misa played it off that Elagabalus was Severus's son. If there's no "legitimate" successor to the severance dynasty, macrinus position is secure. Only the eastern Syrian legions had supported Elagabalus to begin with, the western legions and the senate remained loyal to macrinus until he was dead.