From his perspective it was a rational decision because he didn't see any better one. There is absolutely not mental instability in play here.

My feeling is that most people who commit suicide do it because they decided it on a rational basis. Some people commit suicide because they want to stop a pain they feel and in their view will apparently last forever or will get worse. I.e cancer, someone has to spend the rest of his life in a bed after an accident ... People can also commit suicide as a sacrifice to save others. But I guess you were not talking of these types of suicide.

Suicide due to mental instability is probably just a small fraction of all suicide.

Your claim is typical from someone who didn't took the time and made the effort to understand what can lead someone fully rational to suicide. A rational thinking compares the cost and benefit of dying or living. According to some context a perfectly sain person may rationnaly conclude that commiting suicide is the best choice.

The rational thinking they had from their perspective and knowledge may have been wrong, but this is not to be confused with mental instability.