Unless you're using the word 'murder' in a very broad sense, suicide isn't murder. Murder, by definition, is the killing of another human being by a human being without legal sanction. Suicide might be against your beliefs, but it's not murder.
I would also like to ask - would you hold someone accountable for a murder committed while they were mentally ill? Would such be a sin? For example, imagine a person with a severe psychosis. He genuinely believes that a certain person represents a grave threat to himself or to others (say, he believes this person is going to murder his family). Acting under this delusion, he kills the man, to protect himself/others. Legally, he would be not guilty of murder by reason of mental defect, and the fact that he may have spent considerable time planning it would not change the verdict. Would you hold him accountable? Would he have committed a sin?
Suicide is often the same. People plan it (often, as you stated, taking care to choose as painless a way as possible) long before doing it. However, if they are doing it while suffering from a mental illness (for example, clinical depression), are they to be held accountable? Should we condemn them (or at least their action) or pity them?
I think that once again here you are letting your prejudices about clinical depression (and possibly other mental illnesses) colour your thinking. You seem to think that people with clinical depression can simply 'snap out of it', perhaps by improving their relationships, and so suicide while suffering from it is not defensible, since they could easily have gotten out of it. Forgive me (and correct me) if I'm wrong, but this seems to be your take. And it's simply wrong.
