I do not believe that in ANY situation is suicide an option. Even if the entire world's population were to vanish in an instant, and I was left alone, I would continue to persevere and live out whatever lonely life was ahead of me. If the world was wiped out, it could have been for a reason. Perhaps aliens are conducting a test on the sustainable willpower of humanity, and you have been chosen. Perhaps life is just cruel. Either way, being on Earth could possibly begin humanity again. If we're assuming that the world ends significantly in the future, it could be possible to repopulate the world with advanced genetics, etc. And even if we're talking in the present, suicide would accomplish nothing.