kent sprouse , 42 , faces lethal injection at 6 pm thursday .
in 2002 he gunned down ferris police officer marty steinfeldt , 28 , at a gas station outside of dallas and a customer , pedro moreno , 38 .
sprouse was high on meth , but his insanity defense was rejected .
he was sentenced to death in 2004 .
sprouse has not appealed the sentence recently .
it could be the last execution in texas for a while .
the state has a shortage the lethal drug pentobarbital .