Blue Cross Blue Shield of Massachusetts, seeking to close the book on its widely criticized $11 million payout to former chief executive Cleve L. Killingsworth, yesterday said it will refund $4.2 million to customers, an amount equal to the severance portion of Killingsworth's pay package. The move follows a four-month investigation by the Massachusetts attorney general's office, which found that while Killingsworth was entitled to the money under his contract, such generous severance terms don't serve the purposes of the nonprofit insurer and weaken the authority of its board. Killingsworth, who abruptly quit in March 2010, won't have to give back the severance money or any other part of his payout, which also included a salary and bonus. Blue Cross will deduct the $4.2 million from fourth-quarter earnings, crediting it to employer groups and, in some cases, individual customers, according to senior vice president Jay McQuaide.