"The suicide-prevention movement fears that if suicide is deemed the rational product of someone's mind, we may feel justified in suspecting that mind forever."

Why? If it was rational, then when the reason for it is no longer there, then there is no reason to suspect that mind.

On top of that, if it's rational for one person it's rational for another (assuming the same circumstances), so why suspect the mind in the first place?

Is he using a different meaning of rational? (Perhaps "thought out"?)