UnitedHealth shares plunged 17.4% premarket on Thursday after the insurer dramatically slashed its profit forecast for the year after it revealed disappointing earnings.The healthcare company – whose top insurance executive was shot dead in midtown Manhattan in December – cut its full-year adjusted earnings outlook to a range of $26 to $26.50 per share, down from $29.50 to $30 a share.The firm also reported adjusted income of $7.20 a share in the first quarter, below analysts’ expectations of $7.29, accordi ...