The reason that the perpetrator in that 48 hours episode wore a size other than his own was to throw off the police, obviously. Being a family member of the victims, he knew that he would be looked at closely. He had to use a size different from his own if he hoped to be eliminated as a suspect. This was also JY's reasoning - he's the victim's husband and knew he would be looked at. He's thinking he shuld wear a size 10 when his size is 12 and hope that nobody believes you left the print. The leaving of the size 12 HP print was not planned, obviously.
Keep in mind that a random stranger killer would NOT bother to try to throw off police with a wrong size shoe and then, why would a stranger random killer bother to change out of his bloody shoes before leaving a crime scene??? Nobody would know who he was!! The only people that would try to throw off police by wearing the wrong size shoe is somebody that the police would look at from the get go - somebody that knew the victim. This is common sense.