Djee, people, think logic!
1) a brain is a set of cells. There is a lot that can go wrong during development. If that happens to your body, they call that "physically disabled". If that happens to your brain, why shouldn't you be able to say "mentally challenged"?
2) depending on which disorder, it might or might not be an "excuse". If you can't grab it was wrong (which can be), it's an excuse (autists stripping in public, for example). If you can, than there is no excuse (every pedophile knows that sex with kids is wrong). If they "get off" on insanity, that means they need treatment in order to make them function in society. You can't just kill everybody that's not capable of dealing with society that ends up in prison.
3) there is not one cause of being insane. Generalizing all different patterns into "it's due to religion" is plain stupid. Alcohol abuse of the mother is something else than a trauma from youth-experiences or a genetic defect. There are enough non-religious people insane as well.
This said : a disorder is nothing more than a description of a set of patterns, observed in a person. It's a starting point to get an indication of causes, so you know how to help people finding their way. "I am like that" is no excuse to not try to cope with it.
But : "I am not like that" is no excuse to say that nobody is like that. Every person is unique, and you should be happy that you do function normally in society. It's not because you can walk, that everybody can. It's not because you don't have troubles with fitting in, nobody has them.