Why is suicide always said to lead to negative rebirth when often it is the result of an illness, and not the person's fault?

Buddhism has no compassion the way Christianity does

Christians supplies conditions under which a persons soul can be sentenced to hell for eternity. In Buddhism, no such place, or conditions, exist at all.

Your confusion is in equating "negative karma" to something like ideas of evil in Christianity.

They are not the same. Karma is not good/evil, it is just cause and effect. In Sanskrit, karma just means "action" or "doing". All it really means is that the wheels keeps turning. Every crest has a trough. Each coin has two sides.

In this way, the fundamental Buddhist metaphysic is as compassionate as it can possibly be; it says that nothing is ever wrong. The universe is as it is, because it is. That is all, and that is beautiful.

Christianity, on the other hand, takes almost the opposite approach, saying that things are either gloriously right, or hideously wrong, and that it is the responsibility of each individual to make the right choice.

/r/Buddhism Thread Parent