Gandhi is the perfect pacifist, utterly committed to not bringing about harm to his fellow beings. If a murder pill existed such that it would make murder seem ok without changing any of your other values, Gandhi would refuse to take it on the grounds that he doesn't want his future self to go around doing things that his current self isn't comfortable with. Is there anything you could say to Gandhi that could convince him to take the pill? If a serial killer was hiding under his bed waiting to ambush him, would it be ethical to force him to take it so that he would have a chance to save his own life? If for some convoluted reason he was the only person who could kill the researcher about to complete uFAI, would it be ethical to force him to take the pill so that he'll go and save us all from uFAI?
Charlie is very depressed, utterly certain that life is meaningless and terrible and not going to improve anytime between now and the heat death of the universe. He would kill himself but even that seems pointless. If a magic pill existed that would get rid of depression permanently and without side effects, he would refuse it on the grounds that he doesn't want his future self to go around with a delusion (that everything is fine) which his current self knows to be false. Is there anything you could say to Charlie that could convince him to take it? Would it be ethical to force him to take the pill?
Note: I'm aware of the conventional wisdom for dealing with mental illness, and generally subscribe to it myself. I'm more interested in why people intuitively feel that there's a difference between these two situations, whether there are arguments that could be used to change someone's terminal values, or as a rationale for forcing a change on them.