No, it's not that complicated.

Most people commit suicide because they lose hope that their situation will ever get better. Depression is a key player in suicides. The more depressed people become, the less likely they are to seek help for their depression. They look for a way out of their situation, and realize they'd rather die than live life under the circumstances they foresee for themselves.

I expect Robin Williams killed himself because he knew that Parkinson's disease and his addictions to alcohol and drugs would eventually prevent him from working as an actor, and he could envision no other work that he would be happy doing.