Wanting to CTB is just a thought, and I don't personally believe that a single thought can really constitute a mental illness, though obviously a lot of mental illnesses cause suicidal ideation. Most definitions of illness, mental or physical, require a sort of internal condition on the part of the patient that causes them to suffer. That combination of internal cause and suffering is really what makes something like cancer a disease--not necessarily just that you have a tumor, but that the tumor causes you pain and can kill you, and is within your body. It also makes something like being hit by lightning not a disease, since the cause is external. I think it's extremely debatable how much of mental illnesses throughout a lot of the difficult times in history can be described as an internally caused phenomenon. On the one hand, we might argue that all emotional suffering is internally caused since it is perceived by us. Yet, I doubt somebody who takes that position would then consider someone mentally ill for mourning a loved one's death temporarily, because we accept that there are bounds on the extent of emotional pain we can experience from a specific event that are "normal", and an extent of suffering beyond those bounds that constitutes an illness. I would imagine that someone who considers suicidality to be a mental illness in and of itself assumes implicitly then that never can the bounds of "normal" suffering for any reasonable event be considered to be so high as to include preferring to die, and I just don't think that's true. Our trajectory as a species towards climate crisis, combined with the ever increasing wealth inequality that leaves the majority of our generation less well off than our parents, not to mention the unbearable physical pain of those who are chronically ill, tend to make people more depressed and suicidal the more that they think about it, and insofar as that is the common response, I think it presents an obvious example where being suicidal does not necessarily indicate a mental illness.