Metamorphosis

Sam and Dean discover that Jack Montgomery, a family man, is turning into a Rugaru, a creature that starts as a human but changes into a flesh-eating monster. The people who are infected don't understand what is happening until they turn into cannibals and eat another person. Understanding what it is like to be a victim of a twisted destiny, Sam believes he can reason with Jack and convince him to control his urges. Dean doesn't think Jack's animalistic behavior can be subdued and decides the only honorable thing to do is to shoot him.