Is there a word or phrase for a choice that ultimately leads to tragedy?

I propose fateful, as in Aladdin's fateful decision or Juliet's act on that fateful night.

Although it can mean simply "significant" or "determined by fate," fateful carries connotations of calamity or misfortune.

http://www.merriam-webster.com/dictionary/fateful

1: having a quality of ominous prophecy [a fateful remark] 2 a : involving momentous consequences : decisive [made his fateful decision to declare war — W. L. Shirer] b : deadly, catastrophic

More poetically, to express a stronger sense of menace or wickedness, one might use baneful or baleful.

I propose fatal. It was used in Max Payne 2 video game and just reading the question made me remember this quote:

I had tried to run from it, edit it out. Winterson was dead, I was a murderer. A false start. The hospital bed wasn't the beginning. It was past the point of no return, the fatal choice already made, and it would get worse before the end.

Tragedy has several characteristics, one of which (according to Aristotle) is that the protagonist must choose his fate. Although this is simply a choice, the wrong choice is made based on the character's tragic flaw (hamartia). You could say that the character faces a dilemma, i.e., a choice that yields pitfalls on both sides, but choosing wrong ultimately leads to catastrophe.

I think doomed can also be used:

2a: to fix the fate of : destine

b: to make certain the failure or destruction of

From http://www.merriam-webster.com/dictionary/doomed

Perhaps the word naivete will do.