User blog comment:B214/Question and Answers 4/@comment-120.20.128.230-20141007071346/@comment-24866657-20141009012701

Nope, look at Valerie, she didn't have her whole Longinus ripped off, it was only 1/3 and then what happened? Lived like a corpse, removing the next 1/3 resulted in falling into comatose even after returning back that part for a short time. So no, you remove a person's Sacred Gear, he died, or else Ajuka wouldn't be so serious about rebuilding Issei's artificial body, especially the fact he emphasized on "A chance that the Longinus rejecting the new body."

If it's about reviving someone without their Sacred Gear then it might be like you said, mental deranged, dead man walking, comatose, which, as good as dead.