Why Did Moaning Myrtle Die?

Moaning Myrtle is a ghost of a female student that can be found crying in girls' lavatories. She belongs to Ravenclaw and was killed by Basilisk for being a Muggle-born witch.

Was Moaning Myrtle's death an accident?

I don't think her death was an accident. Tom unleashed the basilisk with the intention for it to kill, and it did. He might not have cast a spell to end her life, but he was directly responsible for how and why she died.

How did Moaning Myrtle die if she was wearing glasses?

She died instantly with her glasses falling into the right position due to the way they were held before and since she had her glasses on when she became a ghost, her glasses were part of the image.