I remember watching the Icaman Chronicles on HBO back in the days which was about Richard kuklinski's story. He was talking about how he would take a target out to lunch and when they got up to use the bathroom or get something he would put cyanide in there food, they would come back eat and be dead soon after, quick simple and easy.
At the end of the trailer that dude praying to God was the one murder he said he regretted. I can't remember the exact details but dude was praying to god to save him, so Richard gave him one hour for God to show him a sign that he should not kill the guy, God didn't intervene so Richard shot him dead.