You are correct, suicide is an illness, therefore it is wrong and irrational.
Why? Because no one in their right or well mind wishes to commit suicide. Well minded people wish to be happy and successful. No one ever wishes to be overwhelmed by misfortune and grief.
However, I agree that most people (including me)have had a spat with this illness at one stage in their lives, and its usually when things don't go our way.
I suppose imperfection can make us involuntarily very unwell if we don't have supportive things to fall back on like positive people and activities.
But I don't think you should hold a grudge against
your fiance, he got mentally unwell, like catching a flu or something. It obviously wasn't his real or well intentions. However its ok to be angry and hate his involuntary unsafe actions. I hope these feelings will pass for you.
We all wish to be invincible against imperfection, illness and harm, but in reality we can only be as vigilant as we possibly can.
The battle against imperfection, unwellness and harm is an ongoing reality for all of us. That's one of the reasons why I became a Christian...