nikko jenkins , 28 , was trying to etch the revelation sign of the beast .
but he now has a series of upside-down 9s across his face .
it is believed he may use the botched case as evidence he is mentally unstable and therefore ineligible to face the death penalty .
jenkins was convicted of shooting dead four people in 10 days after he was released from prison in omaha , nebraska , in 2013 .