Lucius Cornelius Sulla Felix was reprobated because he met and crushed the constitutional challenge of the
populares and got right down in the dirt with them and beat them. He violated the Roman constitution by bringing a standing army inside the walls of the city -- of course, Gaius Marius's followers were merely
slaughtering people in the streets, so I guess we are supposed, even at a distance of 2100 years, not to notice the provocation. The Marians had killed a number of senators, as well as the elderly jurist Scaevola, Cicero's law-teacher and a scholar of great repute, and were dragging people out of their houses and beating them to death.
Sulla's reforms and reconstitution of the state lasted 20 years beyond his death, until the soldier-adventurers rose again (Marius had been one, as had Sulla himself) and formed the First Triumvirate.
Sources: Ronald Syme, The Roman Revolution; Who Was Who in the Roman World, ed. Diana Bowder.
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