kent sprouse , 42 , died of a lethal injection thursday in texas .
it took 22 minutes for sprouse to die after being injected .
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 and he was sentenced to death in 2004 .
` i would like to apologize to the moreno family and the steinfeldt family for all of the trouble i have caused them , ' said sprouse in his final statement .