On its own, "Shut Up, Dr. Phil" is a funny episode, featuring two funny Buffy alums, Charisma Carpenter and James Marsters, as a feuding husband and wife witch couple. Their murders to get at the other are all very grisly and were stimulating to say the least, and the final bit where they reconcile was great. Even though Cordelia and Spike didn't have too many scenes together (and once again I have to wonder what the fifth season of Angel would be like if Cordelia was in it), it's cool to see both of them together.
Viewed, however, in the context of what Dean did to Amy, it doesn't make too much sense. Dean's exact words were that she'd kill again so he had to kill her. Excuse me, Dean, but didn't the two witches just kill four people? Why is this different? Normally I would be fine with something like this because the funny parts of Supernatural are often cordoned off from the rest of the show. But the writers use Don to deal with the Leviathan, bringing the two worlds together. I'm not sure I really want to make a big deal about this, but the writers don't seem to care about these obvious contradictions, which is troubling when the brothers try to talk about morality and stuff like that.