Tracy Abrams added 13 points off the bench for the Illini (19-8, 6-7 Big Ten), who have won four straight.
Reggie Hearn led Northwestern with 11 points. The Wildcats (13-13, 4-9) have lost three straight and five of their last six.
Illinois scored the first 16 points of the second half after ending the first half on a 10-0 run. Richardson's layup extended Illinois' lead to 45-15 with 13:53 remaining. James Montgomery scored Northwestern's first points of the half on a layup with 13:26 left.
Northwestern shot 25 percent from the field and Illinois shot 43.6 percent. Illinois avenged a 68-54 home loss to Northwestern on Jan. 17. During their winning streak, the Illini have also beaten top-ranked Indiana, then-No. 18 Minnesota and Purdue.