Regulus wouldn't say it but he was slightly glad that Sophia was a slut. If she hadn’t ruined your relationship with Sirius, Regulus would never be able to be with you. The two of you would never have been able to fall in love and fix all of the broken parts that the other one had. In some ways, Regulus was thankful for Sophia. He would never say it out loud though. Those thoughts stayed safely guarded in his mind.