In my case I just don't have any interest in suffering for decades just to be tormented by old age and die anyway. And for me suicide is rational as it's the way to prevent all future unnecessary suffering in an existence I never saw as desirable in the first place, suicide is the way to escape from the futile and torturous burden of existing as a human.
There are no disadvantages to not-existing which is the fate we are destined for anyway yet in existence there is unlimited potential for pain, torment and harm. I see existence as very undesirable, harmful and evil, I'd see it as better to not exist regardless of the circumstances.
It disturbs me how there is no limit as to how much a human can suffer, existence truly is an abomination, I'd say that for me my wish to die is a result of becoming aware of how existence itself really is the true problem, I find it tragic how humans so harmfully impose existence in the first place. But it's also down to the fact that I don't want to suffer and see all suffering as best avoided.