Plot hole: Although William Lewis burns his fingertips in an attempt to avoid identification, the Special Victims Unit could have identified him early on by examining the remaining unburned area of Lewis' hands, and in good police procedure still record the newly scarred fingerprints, as these new scars actually make his fingerprints more unique. His scarred fingerprints would still be at Alice's apartment crime scene, and during the trial at the end of the episode the prosecution would still have a strong argument for placing Lewis at Alice's apartment, even if the DNA evidence is thrown out because of alleged cross contamination.
Plot hole: M.E. Warner explains of Lewis and Benson "both their fingerprints are on the gun." Lewis burned his off in a particularly gruesome scene a few episodes earlier. (It was at the beginning of his arc). Only Benson's should be on her gun. (00:29:20)
Answer: It was a misconception about Jehovah's Witnesses beliefs. Only blood transfusions are discouraged in their beliefs, medically.