You usually don’t discuss suicidal thoughts with others. And because of that, they fester inside. Often the people around you have no clue what’s going on with you until it’s too late.
I personally don’t believe suicidal thoughts in themselves are necessarily unhealthy*. In fact, I believe they can be a good way to use your brain the way it was designed:
To consider all outcomes of a situation, good, bad and ugly. Your brain appreciates that it has a choice.