They discuss how the unsub used succinylcholine to paralyze his victims before making it look like they killed themselves. This would be accurate, but it shows scenes where the unsub is talking to the people while paralyzed. Succinylcholine paralyzes ALL skeletal muscles, including the diaphragm that controls breathing. In other words, people who are paralyzed by it can't voluntarily breath. They would have suffocated within minutes of being injected.
It wouldn't matter if a gfci receptacle had been in the bathroom where the space heater was thrown in the tub, he would've died anyway. The house was built in the 1930's, when only a hot and a neutral would be pulled to the junction box, no ground wire. Without the ground the gfci is useless, just for show gfci stands for ground fault circuit inerrupter
The trivia item below may give away important plot points.
The unsub in this episode, played by Scott Michael Campbell also appeared in another crime-solving show, Psych (2006). Both played a serial killer who uses his past tragedy of losing a family member to suicide as a motive to force others to commit suicide.