What you've missed is that the third line doesn't care about whether or not you're pressing shift (or anything else that isn't ctrl), and macros are executed from top to bottom and so it never gets to the fifth line. This should work:
Originally Posted by Dessan
If holding ctrl and holding shift, use Gorefiend's Grasp on focus; else, if holding shift, use Gorefiend's Grasp on player; else, if holding ctrl, use Gorefiend's Grasp on target; else use Death Grip on target:
/use [mod:ctrl,mod:shift,@focus][mod:shift,@player][mod:ctrl]Gorefiend's Grasp;Death Grip
Generally you should try to have bigger groups of conditionals earlier in macros.