Why Sulla general slaughtered inhabitants of Praeneste?

Question

The citizen doesn't seem to be against Sulla. In fact they disallow Marius from coming in.

Its citizens were offered Roman citizenship in 90 BC in the Social War, when concessions had to be made by Rome to cement necessary alliances. In Sulla's second civil war, Gaius Marius the Younger was blockaded in the town by the forces of Sulla (82 BC). When the city was captured, Marius slew himself, the male inhabitants were massacred in cold blood, and a military colony was settled on part of its territory. From an inscription it appears that Sulla delegated the foundation of the new colony to Marcus Terentius Varro Lucullus, who was consul in 73 BC. Within a decade the lands of the colonia had been assembled by a few large landowners.

Answer 1

According to Appian, Praeneste did in fact allow Marius and part of his army into the town, before shutting the gates as Sulla's army arrived and trapping the remainder of Marius's army outside. They continued to hold out until Carbo's reinforcements were destroyed, at which point the town surrendered.

My reading of Appian is that the killing was ordered by Lucretius, who besieged the town, rather than Sulla, who arrived later and only ordered the deaths of the senators that Lucretius had not already killed. Other than those who had been helpful, all Samnite and Praenestian men were killed. However, Romans, women and children were all spared.

Given Praeneste's active support for Marius, it's not surprising that its citizens were thus slaughtered. The Romans, who were spared, were told that they would be pardoned even though they deserved death. Praeneste was also very rich at the time, so another possible consideration would have been the opportunity to give the soldiers a massive payday without leaving behind a mass of men disgruntled by the plunder.