Petunia and Vernon Dursley are also the worst people on the planet. They are the scum of the Earth. If all muggles were like them, Sirius would never have left the Black family and been called a blood traitor, because he would not have defended muggles even in the slightest. Hell, the Dursleys are probably as bad as the Blacks with their prejudice against magic. They are certainly on par with their abusive opinions of punishment, although they seem to have spared their own son from that form of discipline. Sirius wants to wrap his hands around Petunia’s thin neck and squeeze it until she's as red as Vernon had gotten when he learned they were magic. Then he wants to show Vernon what abuse in a magical household feels like.