Wolf-
I think that this is the answer. I do not know how contemporary poskim rule on the subject, but if you’ll look in Mesechta Smachos (a.k.a. Masechta Avel Rabasi, one of the ‘masechtos k’tanos’) it seems clear that one who commits suicide due to emotional stress is not in the category of suicides you mentioned who has the halacha of a mumar. Also, Tosafos in Avoda Zara (18a) says that it is mutar to kill oneself if he is afraid that otherwise someone will torture him to do something wrong and he knows he will not have the strength to hold out. And the gemara over there is clear that in extreme pain that will eventually result in death it may not be so bad if one commits suicide either. It seems apparent that the category of suicides you refer to includes only a specific type, namely those who are emotionally stable yet do not believe their life has a purpose. Not every murderer is making such a ‘lofty’ claim when he kills someone, because he may not be thinking that much. But someone of sound mind who makes the decision to kill himself must have thought long and hard and come to the conclusion that his life has no purpose, and that is why it is such a terrible act. It is a denial of everything the Torah stands for.