That's an easy argument but I'm not sure it's a correct one. The answers to the following questions may be different:

• Is it a good thing on the whole that A exists?
• Now that A already exists, does A want to commit suicide?

for at least the following reasons:

• Other people will be negatively affected (it may be worse to get to know someone and have them commit suicide, than never to encounter them at all).
• People have an inbuilt preference for staying alive, for obvious biological reasons, so someone may go on wanting to live well beyond the point at which each day of life is substantially negative for them. (Just as someone may go on wanting to take heroin well beyond the point at which they'd be happier if they stopped.)