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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.