Ill-fated twins pop up all over the place, often enough that you'd THINK being born a twin would immediately start people panicking over the future results.
Romulus and Remus
Growing up they were heroes (according to Plutarch) and so honorable that after they helped Numitor (their grandfather) win back his kingdom from his-brother-their-uncle-who-deposed-him, they set off at once so that they wouldn't pose a threat to Numitor's rule. When they began founding their own city, however, the trouble started. Romulus and Remus both had different ideas for how Rome should be built and determined to settle it FAIRLY by letting the gods decide via the number of birds in the sky. Remus saw 6 Vultures. Romulus claimed to have seen 12. Remus, believing his brother had lied, began obstructing the building of the city wall by Romulus's design, and generally made a problem about it. He jumped over the wall, and was struck down, either by Romulus, or by Romulus's friend Celer. Either way, Remus ended up dead.
Tragedy Factor: 4 out of 5 if Romulus committed the murder, 3 out of 5 if his buddy Celer did. If Romulus lied about the vultures, then killed his brother for calling him on it, he's a real piece of work. I'm not sure I'd be too proud of him as a founder of MY city. Remus really got a raw deal.
Castor and Pollux
Absolutely inseparable twins, also known as the Dioscuri, and brothers to Helen of Sparta/Troy. Pollux was a son of Zeus and Castor was a son of Tyndareus (presumably--more on their paternity issues HERE). Castor and Pollux were by all accounts incredible hunters, horsemen, and very admirable men, running about the Greek countryside performing heroic deeds, raiding cattle, and stealing women (in the great tradition of all Greek heroes!), including the rescue of Helen from Theseus's clutches (or something). On one of these expeditions, they were caught in the act of raiding and Castor was killed by a spear. Pollux avenged his brother's death by killing the men who fought them, and then may or may not have been saved from death himself by Zeus-his-father. After this, Zeus offered Pollux the choice of giving Castor half his immortality, or to leave his brother and join the gods on Olympus. Pollux chose to share his immortality, so that he would never be parted from his brother. (can I get an "Awwww!")
Tragedy Factor: No more than a 2. Yeah, it's sad that Castor died, but that's the risk you take when you're stealing other people's livestock! Besides, they got a happy ending. Who can REALLY complain about spending half your time partying together on Olympus, even if the other half is spent in Hades? But see, this is why I do not understand how Theseus didn't get a sweet afterlife offer! I guess only the sons of Zeus get the hook-up to the Olympus after-party.
Any other famous and ill-fated twins that you can think of?