I think what happened to you was that the person you gripped was chasing you or running in your direction, and you were jumping off the cliff and were able to Death Grip because your avatar was still considered over ground the moment you casted it, and as you continued to fall, they continued to walk forward, thus falling off the cliff with you, even though they would've been fine if they just stopped moving.
Similar to a rogue casting a good distract, sometimes that can get keyboard turners to walk right off cliffs before they notice they were turned.
Really exploits like these require videos. Sure I'm not giving any evidence that this couldn't work, I'm just giving past examples of proper game mechanics. But for the 3 people or w/e that have said this works--make a video? +rep to that.